Encoder, decoder, encoding method, and decoding method

ABSTRACT

An encoder includes memory and circuitry. The circuitry: derives a first motion vector in a unit of a prediction block using a first inter frame prediction mode that uses a degree of matching between two reconstructed images of two regions in two difference pictures, the prediction block being obtained by splitting an image included in a video; and performs, in the unit of the prediction block, a first motion compensation process that generates a prediction image by referring to a spatial gradient of luminance in an image generated by performing motion compensation using the first motion vector derived.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a U.S. continuation application of PCT InternationalPatent Application Number PCT/JP2018/018444 filed on May 14, 2018,claiming the benefit of priority of U.S. Provisional Patent ApplicationNo. 62/508,517 filed on May 19, 2017, the entire contents of which arehereby incorporated by reference.

BACKGROUND 1. Technical Field

The present disclosure relates to an encoder, a decoder, an encodingmethod, and a decoding method.

2. Description of the Related Art

There has been H.265 as a standard for coding a video. H.265 is alsoreferred to as high efficiency video coding (HEVC) (see 11.265 (TSO/IEC23008-2 HEVC (High Efficiency Video Coding)), for example).

SUMMARY

An encoder according to one aspect of the present disclosure includescircuitry and memory. Using the memory, the circuitry: derives a firstmotion vector in a unit of a prediction block using a first inter frameprediction mode that uses a degree of matching between two reconstructedimages of two regions in two difference pictures, the prediction blockbeing obtained by splitting an image include in a video; and performs,in the unit of the prediction block, a first motion compensation processthat generates a prediction image by referring to a spatial gradient ofluminance in an image generated by performing motion compensation usingthe first motion vector derived.

A decoder according to one aspect of the present disclosure includescircuitry and memory. Using the memory, the circuitry: derives a firstmotion vector in a unit of a prediction block using a first inter frameprediction mode that uses a degree of matching between two reconstructedimages of two regions in two difference pictures, the prediction blockbeing obtained by splitting an image include in a video; and performs,in the unit of the prediction block, a first motion compensation processthat generates a prediction image by referring to a spatial gradient ofluminance in an image generated by performing motion compensation usingthe first motion vector derived.

It should be noted that these general or specific aspects may beimplemented by a system, a device, a method, an integrated circuit, acomputer program, or a non-transitory computer-readable recording mediumsuch as a compact disc read only memory (CD-ROM), or by any combinationof systems, devices, methods, integrated circuits, computer programs, orrecording media.

BRIEF DESCRIPTION OF DRAWINGS

These and other objects, advantages and features of the disclosure willbecome apparent from the following description thereof taken inconjunction with the accompanying drawings that illustrate a specificembodiment of the present disclosure.

FIG. 1 is a block diagram illustrating a functional configuration of anencoder according to Embodiment 1.

FIG. 2 illustrates one example of block splitting according toEmbodiment 1.

FIG. 3 is a chart indicating transform basis functions for eachtransform type.

FIG. 4A illustrates one example of a filter shape used in ALF.

FIG. 4B illustrates another example of a filter shape used in ALF.

FIG. 4C illustrates another example of a filter shape used in ALF.

FIG. 5A illustrates 67 intra prediction modes used in intra prediction.

FIG. 5B is a flow chart for illustrating an outline of a predictionimage correction process performed via OBMC processing.

FIG. 5C is a conceptual diagram for illustrating an outline of aprediction image correction process performed via OBMC processing.

FIG. 5D illustrates one example of FRUC.

FIG. 6 is for illustrating pattern matching (bilateral matching) betweentwo blocks along a motion trajectory.

FIG. 7 is for illustrating pattern matching (template matching) betweena template in the current picture and a block in a reference picture.

FIG. 8 is for illustrating a model assuming uniform linear motion.

FIG. 9A is for illustrating deriving a motion vector of each sub-blockbased on motion vectors of neighboring blocks.

FIG. 9B is for illustrating an outline of a process for deriving amotion vector via merge mode.

FIG. 9C is a conceptual diagram for illustrating an outline of DMVRprocessing.

FIG. 9D is for illustrating an outline of a prediction image generationmethod using a luminance correction process performed via LICprocessing.

FIG. 10 is a block diagram illustrating a functional configuration of adecoder according to Embodiment 1.

FIG. 11 is a flow chart illustrating inter frame prediction according toComparative Example 1.

FIG. 12 is a flow chart illustrating inter frame prediction according toComparative Example 2.

FIG. 13 is a flow chart illustrating inter frame prediction according toEmbodiment 1.

FIG. 14 is a conceptual diagram illustrating a template FRUC modeaccording to Embodiment 1.

FIG. 15 is a conceptual diagram illustrating a bilateral FRUC modeaccording to Embodiment 1.

FIG. 16 is a flow chart illustrating operations of deriving a motionvector in a FRUC mode according to Embodiment 1.

FIG. 17 is a conceptual diagram illustrating BIO according to Embodiment1.

FIG. 18 is a flow chart illustrating BIG according to Embodiment 1.

FIG. 19 is a block diagram illustrating an implementation example of anencoder according to Embodiment 1.

FIG. 20 is a block diagram illustrating an implementation example of adecoder according to Embodiment 1.

FIG. 21 illustrates an overall configuration of a content providingsystem for implementing a content distribution service.

FIG. 22 illustrates one example of an encoding structure in scalableencoding.

FIG. 23 illustrates one example of an encoding structure in scalableencoding.

FIG. 24 illustrates an example of a display screen of a web page.

FIG. 25 illustrates an example of a display screen of a web page.

FIG. 26 illustrates an example of a smartphone.

FIG. 27 is a block diagram illustrating a configuration example of asmartphone.

DETAILED DESCRIPTION OF THE EMBODIMENTS

An encoder according to one aspect of the present disclosure includescircuitry and memory. Using the memory, the circuitry: derives a firstmotion vector in a unit of a prediction block using a first inter frameprediction mode that uses a degree of matching between two reconstructedimages of two regions in two difference pictures, the prediction blockbeing obtained by splitting an image include in a video; and performs,in the unit of the prediction block, a first motion compensation processthat generates a prediction image by referring to a spatial gradient ofluminance in an image generated by performing motion compensation usingthe first motion vector derived.

With this, the encoder can reduce an amount of processing by performingthe derivation of the motion vector using the first inter frameprediction mode, and the first motion compensation process in the unitof the prediction block, compared to a case in which the encoderperforms these processes in a unit of a sub-block. Further, since thefirst compensation process that generates the prediction image byreferring to the spatial gradient of the luminance can make correctionin a unit finer than the unit of the prediction block, it is possible tosuppress a decrease in coding efficiency when the processing is notperformed in the unit of the sub-block. Accordingly, the encoder canreduce the amount of processing while suppressing a decrease in codingefficiency.

For example, using the memory, the circuitry may further: derive asecond motion vector in the unit of the prediction block using a secondinter frame prediction mode that uses a degree of matching between acurrent prediction block and a reconstructed image of a region includedin a reference picture; perform, in the unit of the prediction block, asecond motion compensation process that generates a prediction image byreferring to a spatial gradient of luminance in an image generated byperforming motion compensation using the second motion vector derived;and generate an encoded bitstream including information for identifyingthe second motion vector.

With this, it is possible to make a processing unit of motioncompensation identical between when the first inter frame predictionmode is used and when the second inter frame prediction mode is used.Accordingly, it is possible to simplify the implementation of motioncompensation.

For example, the two regions in the first inter frame prediction modemay be (i) a region in a current picture neighboring a currentprediction block and a region in a reference picture or (ii) two regionsin two different reference pictures.

A decoder according to one aspect of the present disclosure includescircuitry and memory. Using the memory, the circuitry: derives a firstmotion vector in a unit of a prediction block using a first inter frameprediction mode that uses a degree of matching between two reconstructedimages of two regions in two difference pictures, the prediction blockbeing obtained by splitting an image include in a video; and performs,in the unit of the prediction block, a first motion compensation processthat generates a prediction image by referring to a spatial gradient ofluminance in an image generated by performing motion compensation usingthe first motion vector derived.

With this, the decoder can reduce an amount of processing by performingthe derivation of the motion vector using the first inter frameprediction mode, and the first motion compensation process in the unitof the prediction block, compared to a case in which the decoderperforms these processes in a unit of a sub-block. Further, since thefirst compensation process that generates the prediction image byreferring to the spatial gradient of the luminance can make correctionin a unit finer than the unit of the prediction block, it is possible tosuppress a decrease in coding efficiency when the processing is notperformed in the unit of the sub-block. Accordingly, the decoder canreduce the amount of processing while suppressing a decrease in codingefficiency.

For example, using the memory, the circuitry may further: obtain, froman encoded bitstream, information for identifying a second motion vectorin the unit of the prediction block; derive the second motion vector inthe unit of the prediction block using a second inter frame predictionmode that uses the information; and perform, in the unit of theprediction block, a second motion compensation process that generates aprediction image by referring to a spatial gradient of luminance in animage generated by performing motion compensation using the secondmotion vector derived.

With this, it is possible to make a processing unit of motioncompensation identical between when the first inter frame predictionmode is used and when the second inter frame prediction mode is used.Accordingly, it is possible to simplify the implementation of motioncompensation.

For example, the two regions in the first inter frame prediction modemay be (i) a region in a current picture neighboring a currentprediction block and a region in a reference picture or (ii) two regionsin two different reference pictures.

An encoding method according to one aspect of the present disclosureincludes: deriving a first motion vector in a unit of a prediction blockusing a first inter frame prediction mode that uses a degree of matchingbetween two reconstructed images of two regions in two differentpictures, the prediction block being obtained by splitting an imageincluded in a video; and performing, in the unit of the predictionblock, a first motion compensation process that generates a predictionimage by referring to a spatial gradient of luminance in an imagegenerated by performing motion compensation using the first motionvector derived.

With this, the encoding method makes it possible to reduce an amount ofprocessing by performing the derivation of the motion vector using thefirst inter frame prediction mode, and the first motion compensationprocess in the unit of the prediction block, compared to a case in whichthese processes are performed in a unit of a sub-block. Further, sincethe first compensation process that generates the prediction image byreferring to the spatial gradient of the luminance can make correctionin a unit finer than the unit of the prediction block, it is possible tosuppress a decrease in coding efficiency when the processing is notperformed in the unit of the sub-block. Accordingly, the encoding methodmakes it possible to reduce the amount of processing while suppressing adecrease in coding efficiency.

A decoding method according to one aspect of the present disclosureincludes: deriving a first motion vector in a unit of a prediction blockusing a first inter frame prediction mode that uses a degree of matchingbetween two reconstructed images of two regions in two differentpictures, the prediction block being obtained by splitting an imageincluded in a video; and performing, in the unit of the predictionblock, a first motion compensation process that generates a predictionimage by referring to a spatial gradient of luminance in an imagegenerated by performing motion compensation using the first motionvector derived.

With this, the decoding method makes it possible to reduce an amount ofprocessing by performing the derivation of the motion vector using thefirst inter frame prediction mode, and the first motion compensationprocess in the unit of the prediction block, compared to a case in whichthese processes are performed in a unit of a sub-block. Further, sincethe first compensation process that generates the prediction image byreferring to the spatial gradient of the luminance can make correctionin a unit finer than the unit of the prediction block, it is possible tosuppress a decrease in coding efficiency when the processing is notperformed in the unit of the sub-block. Accordingly, the decoding methodmakes it possible to reduce the amount of processing while suppressing adecrease in coding efficiency.

Furthermore, these general or specific aspects may be implemented by asystem, a device, a method, an integrated circuit, a computer program,or a non-transitory computer-readable recording medium such as a compactdisc read only memory (CD-ROM), or by any combination of systems,devices, methods, integrated circuits, computer programs, or recordingmedia.

Hereinafter, embodiments will be described with reference to thedrawings.

Note that the embodiments described below each show a general orspecific example. The numerical values, shapes, materials, components,the arrangement and connection of the components, steps, order of thesteps, etc., indicated in the following embodiments are mere examples,and therefore are not intended to limit the scope of the claims.Therefore, among the components in the following embodiments, those notrecited in any of the independent claims defining the broadest inventiveconcepts are described as optional components.

Embodiment 1

First, an outline of Embodiment 1 will be presented. Embodiment 1 is oneexample of an encoder and a decoder to which the processes and/orconfigurations presented in subsequent description of aspects of thepresent disclosure are applicable. Note that Embodiment 1 is merely oneexample of an encoder and a decoder to which the processes and/orconfigurations presented in the description of aspects of the presentdisclosure are applicable. The processes and/or configurations presentedin the description of aspects of the present disclosure can also beimplemented in an encoder and a decoder different from those accordingto Embodiment 1.

When the processes and/or configurations presented in the description ofaspects of the present disclosure are applied to Embodiment 1, forexample, any of the following may be performed.

(1) regarding the encoder or the decoder according to Embodiment 1,among components included in the encoder or the decoder according toEmbodiment 1, substituting a component corresponding to a componentpresented in the description of aspects of the present disclosure with acomponent presented in the description of aspects of the presentdisclosure;

(2) regarding the encoder or the decoder according to Embodiment 1,implementing discretionary changes to functions or implemented processesperformed by one or more components included in the encoder or thedecoder according to Embodiment 1, such as addition, substitution, orremoval, etc., of such functions or implemented processes, thensubstituting a component corresponding to a component presented in thedescription of aspects of the present disclosure with a componentpresented in the description of aspects of the present disclosure;

(3) regarding the method implemented by the encoder or the decoderaccording to Embodiment 1, implementing discretionary changes such asaddition of processes and/or substitution, removal of one or more of theprocesses included in the method, and then substituting a processescorresponding to a process presented in the description of aspects ofthe present disclosure with a process presented in the description ofaspects of the present disclosure;

(4) combining one or more components included in the encoder or thedecoder according to Embodiment 1 with a component presented in thedescription of aspects of the present disclosure, a component includingone or more functions included in a component presented in thedescription of aspects of the present disclosure, or a component thatimplements one or more processes implemented by a component presented inthe description of aspects of the present disclosure;

(5) combining a component including one or more functions included inone or more components included in the encoder or the decoder accordingto Embodiment 1, or a component that implements one or more processesimplemented by one or more components included in the encoder or thedecoder according to Embodiment 1 with a component presented in thedescription of aspects of the present disclosure, a component includingone or more functions included in a component presented in thedescription of aspects of the present disclosure, or a component thatimplements one or more processes implemented by a component presented inthe description of aspects of the present disclosure;

(6) regarding the method implemented by the encoder or the decoderaccording to Embodiment 1, among processes included in the method,substituting a process corresponding to a process presented in thedescription of aspects of the present disclosure with a processpresented in the description of aspects of the present disclosure; and

(7) combining one or more processes included in the method implementedby the encoder or the decoder according to Embodiment 1 with a processpresented in the description of aspects of the present disclosure.

Note that the implementation of the processes and/or configurationspresented in the description of aspects of the present disclosure is notlimited to the above examples. For example, the processes and/orconfigurations presented in the description of aspects of the presentdisclosure may be implemented in a device used for a purpose differentfrom the moving picture/picture encoder or the moving picture/picturedecoder disclosed in Embodiment 1. Moreover, the processes and/orconfigurations presented in the description of aspects of the presentdisclosure may be independently implemented. Moreover, processes and/orconfigurations described in different aspects may be combined.

[Encoder Outline]

First, the encoder according to Embodiment 1 will be outlined. FIG. 1 isa block diagram illustrating a functional configuration of encoder 100according to Embodiment 1. Encoder 100 is a moving picture/pictureencoder that encodes a moving picture/picture block by block.

As illustrated in FIG. 1, encoder 100 is a device that encodes a pictureblock by block, and includes splitter 102, subtractor 104, transformer106, quantizer 108, entropy encoder 110, inverse quantizer 112, inversetransformer 114, adder 116, block memory 118, loop filter 120, framememory 122, intra predictor 124, inter predictor 126, and predictioncontroller 128.

Encoder 100 is realized as, for example, a generic processor and memory.In this case, when a software program stored in the memory is executedby the processor, the processor functions as splitter 102, subtractor104, transformer 106, quantizer 1.08, entropy encoder 110, inversequantizer 112, inverse transformer 1.1.4, adder 116, loop filter 120,intra predictor 124, inter predictor 126, and prediction controller 128.Alternatively, encoder 100 may be realized as one or more dedicatedelectronic circuits corresponding to splitter 102, subtractor 104,transformer 106, quantizer 108, entropy encoder 11.0, inverse quantizer112, inverse transformer 114, adder 116, loop filter 120, intrapredictor 124, inter predictor 126, and prediction controller 128.

Hereinafter, each component included in encoder 100 will be described.

[Splitter]

Splitter 102 splits each picture included in an input moving pictureinto blocks, and outputs each block to subtractor 104. For example,splitter 102 first splits a picture into blocks of a fixed size (forexample, 128×128). The fixed size block is also referred to as codingtree unit (CTU). Splitter 102 then splits each fixed size block intoblocks of variable sizes (for example, 64×64 or smaller), based onrecursive quadtree and/or binary tree block splitting. The variable sizeblock is also referred to as a coding unit (CU), a prediction unit (PU),or a transform unit (TU). Note that in this embodiment, there is no needto differentiate between CU, PU, and TU; all or some of the blocks in apicture may be processed per CU, PU, or TU.

FIG. 2 illustrates one example of block splitting according toEmbodiment 1. In FIG. 2, the solid lines represent block boundaries ofblocks split by quadtree block splitting, and the dashed lines representblock boundaries of blocks split by binary tree block splitting.

Here, block 10 is a square 128×128 pixel block (128×128 block). This128×128 block 10 is first split into four square 64×64 blocks (quadtreeblock splitting).

The top left 64×64 block is further vertically split into two rectangle32×64 blocks, and the left 32×64 block is further vertically split intotwo rectangle 1.6×64 blocks (binary tree block splitting). As a result,the top left 64×64 block is split into two 16×64 blocks 11 and 12 andone 32×64 block 13. The top right 64×64 block is horizontally split intotwo rectangle 64×32 blocks 14 and 15 (binary tree block splitting).

The bottom left 64×64 block is first split into four square 32×32 blocks(quadtree block splitting). The top left block and the bottom rightblock among the four 32×32 blocks are further split. The top left 32×32block is vertically split into two rectangle 16×32 blocks, and the right16×32 block is further horizontally split into two 16×16 blocks (binarytree block splitting). The bottom right 32×32 block is horizontallysplit into two 32×16 blocks (binary tree block splitting). As a result,the bottom left 64×64 block is split into 16×32 block 16, two 16×16blocks 17 and 1.8, two 32×32 blocks 19 and 20, and two 32×16 blocks 21and 22.

The bottom right 64×64 block 23 is not split.

As described above, in FIG. 2, block 10 is split into 13 variable sizeblocks 11 through 23 based on recursive quadtree and binary tree blocksplitting. This type of splitting is also referred to as quadtree plusbinary tree (QTBT) splitting.

Note that in FIG. 2, one block is split into four or two blocks(quadtree or binary tree block splitting), but splitting is not limitedto this example. For example, one block may be split into three blocks(ternary block splitting). Splitting including such ternary blocksplitting is also referred to as multi-type tree (MBT) splitting.

[Subtractor]

Subtractor 104 subtracts a prediction signal (prediction sample) from anoriginal signal (original sample) per block split by splitter 102. Inother words, subtractor 104 calculates prediction errors (also referredto as residuals) of a block to be encoded (hereinafter referred to as acurrent block). Subtractor 104 then outputs the calculated predictionerrors to transformer 106.

The original signal is a signal input into encoder 100, and is a signalrepresenting an image for each picture included in a moving picture (forexample, a luma signal and two chroma signals). Hereinafter, a signalrepresenting an image is also referred to as a sample.

[Transformer]

Transformer 106 transforms spatial domain prediction errors intofrequency domain transform coefficients, and outputs the transformcoefficients to quantizer 108. More specifically, transformer 106applies, for example, a predefined discrete cosine transform (DCT) ordiscrete sine transform (DST) to spatial domain prediction errors.

Note that transformer 106 may adaptively select a transform type fromamong a plurality of transform types, and transform prediction errorsinto transform coefficients by using a transform basis functioncorresponding to the selected transform type. This sort of transform isalso referred to as explicit multiple core transform (EMT) or adaptivemultiple transform (AMT).

The transform types include, for example, DCT-II, DCT-V, DCT-VIII,DST-I, and DST-VII. FIG. 3 is a chart indicating transform basisfunctions for each transform type. In FIG. 3. N indicates the number ofinput pixels. For example, selection of a transform type from among theplurality of transform types may depend on the prediction type (intraprediction and inter prediction), and may depend on intra predictionmode.

Information indicating whether to apply such EMT or AMT (referred to as,for example, an AMT flag) and information indicating the selectedtransform type is signalled at the CU level. Note that the signaling ofsuch information need not be performed at the CU level, and may beperformed at another level (for example, at the sequence level, picturelevel, slice level, tile level, or CTU level).

Moreover, transformer 106 may apply a secondary transform to thetransform coefficients (transform result). Such a secondary transform isalso referred to as adaptive secondary transform (AST) or non-separablesecondary transform (NSST). For example, transformer 106 applies asecondary transform to each sub-block (for example, each 4×4 sub-block)included in the block of the transform coefficients corresponding to theintra prediction errors. Information indicating whether to apply NSSTand information related to the transform matrix used in NSST aresignalled at the CU level. Note that the signaling of such informationneed not be performed at the CU level, and may be performed at anotherlevel (for example, at the sequence level, picture level, slice level,tile level, or cru level).

Here, a separable transform is a method in which a transform isperformed a plurality of times by separately performing a transform foreach direction according to the number of dimensions input. Anon-separable transform is a method of performing a collective transformin which two or more dimensions in a multidimensional input arecollectively regarded as a single dimension.

In one example of a non-separable transform, when the input is a 4×4block, the 4×4 block is regarded as a single array including 16components, and the transform applies a 16×16 transform matrix to thearray.

Moreover, similar to above, after an input 4×4 block is regarded as asingle array including 16 components, a transform that performs aplurality of Givens rotations on the array (i.e., a Hypercube-GivensTransform) is also one example of a non-separable transform.

[Quantizer]

Quantizer 108 quantizes the transform coefficients output fromtransformer 106. More specifically, quantizer 108 scans, in apredetermined scanning order, the transform coefficients of the currentblock, and quantizes the scanned transform coefficients based onquantization parameters (QP) corresponding to the transformcoefficients. Quantizer 108 then outputs the quantized transformcoefficients (hereinafter referred to as quantized coefficients) of thecurrent block to entropy encoder 110 and inverse quantizer 112.

A predetermined order is an order for quantizing/inverse quantizingtransform coefficients. For example, a predetermined scanning order isdefined as ascending order of frequency (from low to high frequency) ordescending order of frequency (from high to low frequency).

A quantization parameter is a parameter defining a quantization stepsize (quantization width). For example, if the value of the quantizationparameter increases, the quantization step size also increases. In otherwords, if the value of the quantization parameter increases, thequantization error increases.

[Entropy Encoder]

Entropy encoder 110 generates an encoded signal (encoded bitstream) byvariable length encoding quantized coefficients, which are inputs fromquantizer 108. More specifically, entropy encoder 110, for example,binarizes quantized coefficients and arithmetic encodes the binarysignal.

[Inverse Quantizer]

Inverse quantizer 112 inverse quantizes quantized coefficients, whichare inputs from quantizer 108. More specifically, inverse quantizer 112inverse quantizes, in a predetermined scanning order, quantizedcoefficients of the current block. Inverse quantizer 112 then outputsthe inverse quantized transform coefficients of the current block toinverse transformer 114.

[Inverse Transformer]

Inverse transformer 114 restores prediction errors by inversetransforming transform coefficients, which are inputs from inversequantizer 112. More specifically, inverse transformer 114 restores theprediction errors of the current block by applying an inverse transformcorresponding to the transform applied by transformer 106 on thetransform coefficients. Inverse transformer 114 then outputs therestored prediction errors to adder 1.16.

Note that since information is lost in quantization, the restoredprediction errors do not match the prediction errors calculated bysubtractor 104. In other words, the restored prediction errors includequantization errors.

[Adder]

Adder 116 reconstructs the current block by summing prediction errors,which are inputs from inverse transformer 114, and prediction samples,which are inputs from prediction controller 128. Adder 116 then outputsthe reconstructed block to block memory 118 and loop filter 120. Areconstructed block is also referred to as a local decoded block.

[Block Memory]

Block memory 118 is storage for storing blocks in a picture to beencoded (hereinafter referred to as a current picture) for reference inintra prediction. More specifically, block memory 118 storesreconstructed blocks output from adder 1.16.

[Loop Filter]

Loop filter 120 applies a loop filter to blocks reconstructed by adder116, and outputs the filtered reconstructed blocks to frame memory 122.A loop filter is a filter used in an encoding loop (in-loop filter), andincludes, for example, a deblocking filter (DF), a sample adaptiveoffset (SAO), and an adaptive loop filter (ALF).

In ALF, a least square error filter for removing compression artifactsis applied. For example, one filter from among a plurality of filters isselected for each 2×2 sub-block in the current block based on directionand activity of local gradients, and is applied.

More specifically, first, each sub-block (for example, each 2×2sub-block) is categorized into one out of a plurality of classes (forexample, 15 or 25 classes). The classification of the sub-block is basedon gradient directionality and activity. For example, classificationindex C is derived based on gradient directionality D (for example, 0 to2 or 0 to 4) and gradient activity A (for example, 0 to 4) (for example,C=5D+A). Then, based on classification index C, each sub-block iscategorized into one out of a plurality of classes (for example, 15 or25 classes).

For example, gradient directionality D is calculated by comparinggradients of a plurality of directions (for example, the horizontal,vertical, and two diagonal directions). Moreover, for example, gradientactivity A is calculated by summing gradients of a plurality ofdirections and quantizing the sum.

The filter to be used for each sub-block is determined from among theplurality of filters based on the result of such categorization.

The filter shape to be used in ALF is, for example, a circular symmetricfilter shape. FIG. 4A through FIG. 4C illustrate examples of filtershapes used in ALF. FIG. 4A illustrates a 5×5 diamond shape filter, FIG.4B illustrates a 7×7 diamond shape filter, and FIG. 4C illustrates a 9×9diamond shape filter. Information indicating the filter shape issignalled at the picture level. Note that the signaling of informationindicating the filter shape need not be performed at the picture level,and may be performed at another level (for example, at the sequencelevel, slice level, tile level, CTU level, or CU level).

The enabling or disabling of ALF is determined at the picture level orCU level. For example, for luma, the decision to apply ALF or not isdone at the CU level, and for chroma, the decision to apply ALF or notis done at the picture level. Information indicating whether ALF isenabled or disabled is signalled at the picture level or CU level. Notethat the signaling of information indicating whether ALF is enabled ordisabled need not be performed at the picture level or CU level, and maybe performed at another level (for example, at the sequence level, slicelevel, tile level, or CTU level).

The coefficients set for the plurality of selectable filters (forexample, 15 or 25 filters) is signalled at the picture level. Note thatthe signaling of the coefficients set need not be performed at thepicture level, and may be performed at another level (for example, atthe sequence level, slice level, tile level, CTU level, CU level, orsub-block level).

[Frame Memory]

Frame memory 122 is storage for storing reference pictures used in interprediction, and is also referred to as a frame buffer. Morespecifically, frame memory 122 stores reconstructed blocks filtered byloop filter 120.

[Intra Predictor]

Intra predictor 124 generates a prediction signal (intra predictionsignal) by intra predicting the current block with reference to a blockor blocks in the current picture and stored in block memory 118 (alsoreferred to as intra frame prediction). More specifically, intrapredictor 124 generates an intra prediction signal by intra predictionwith reference to samples (for example, luma and/or chroma values) of ablock or blocks neighboring the current block, and then outputs theintra prediction signal to prediction controller 128.

For example, intra predictor 124 performs intra prediction by using onemode from among a plurality of predefined intra prediction modes. Theintra prediction modes include one or more non-directional predictionmodes and a plurality of directional prediction modes.

The one or more non-directional prediction modes include, for example,planar prediction mode and DC prediction mode defined in theH.265/high-efficiency video coding (HEVC) standard (see NPL 1).

The plurality of directional prediction modes include, for example, the33 directional prediction modes defined in the H.265/HEVC standard. Notethat the plurality of directional prediction modes may further include32 directional prediction modes in addition to the 33 directionalprediction modes (for a total of 65 directional prediction modes). FIG.5A illustrates 67 intra prediction modes used in intra prediction (twonon-directional prediction modes and 65 directional prediction modes).The solid arrows represent the 33 directions defined in the H.265/HEVCstandard, and the dashed arrows represent the additional 32 directions.

Note that a luma block may be referenced in chroma block intraprediction. In other words, a chroma component of the current block maybe predicted based on a luma component of the current block. Such intraprediction is also referred to as cross-component linear model (CCLM)prediction. Such a chroma block intra prediction mode that references aluma block (referred to as, for example, CCLM mode) may be added as oneof the chroma block intra prediction modes.

Intra predictor 124 may correct post-intra-prediction pixel values basedon horizontal/vertical reference pixel gradients. Intra predictionaccompanied by this sort of correcting is also referred to as positiondependent intra prediction combination (PDPC). Information indicatingwhether to apply PDPC or not (referred to as, for example, a PDPC flag)is, for example, signalled at the CU level. Note that the signaling ofthis information need not be performed at the CU level, and may beperformed at another level (for example, on the sequence level, picturelevel, slice level, tile level, or CTU level).

[Inter Predictor]

Inter predictor 126 generates a prediction signal (inter predictionsignal) by inter predicting the current block with reference to a blockor blocks in a reference picture, which is different from the currentpicture and is stored in frame memory 122 (also referred to as interframe prediction). Inter prediction is performed per current block orper sub-block (for example, per 4×4 block) in the current block. Forexample, inter predictor 126 performs motion estimation in a referencepicture for the current block or sub-block. Inter predictor 126 thengenerates an inter prediction signal of the current block or sub-blockby motion compensation by using motion information (for example, amotion vector) obtained from motion estimation. Inter predictor 126 thenoutputs the generated inter prediction signal to prediction controller128.

The motion information used in motion compensation is signalled. Amotion vector predictor may be used for the signaling of the motionvector. In other words, the difference between the motion vector and themotion vector predictor may be signalled.

Note that the inter prediction signal may be generated using motioninformation for a neighboring block in addition to motion informationfor the current block obtained from motion estimation. Morespecifically, the inter prediction signal may be generated per sub-blockin the current block by calculating a weighted sum of a predictionsignal based on motion information obtained from motion estimation and aprediction signal based on motion information for a neighboring block.Such inter prediction (motion compensation) is also referred to asoverlapped block motion compensation (OBMC).

In such an OBMC mode, information indicating sub-block size for OBMC(referred to as, for example, OBMC block size) is signalled at thesequence level. Moreover, information indicating whether to apply theOBMC mode or not (referred to as, for example, an OBMC flag) issignalled at the CU level. Note that the signaling of such informationneed not be performed at the sequence level and CU level, and may beperformed at another level (for example, at the picture level, slicelevel, tile level, CTU level, or sub-block level).

Hereinafter, the OBMC mode will be described in further detail. FIG. 5Bis a flowchart and FIG. 5C is a conceptual diagram for illustrating anoutline of a prediction image correction process performed via OBMCprocessing.

First, a prediction image (Pred) is obtained through typical motioncompensation using a motion vector (MV) assigned to the current block.

Next, a prediction image (Pred_L) is obtained by applying a motionvector (MV_L) of the encoded neighboring left block to the currentblock, and a first pass of the correction of the prediction image ismade by superimposing the prediction image and Pred_L.

Similarly, a prediction image (Pred_U) is obtained by applying a motionvector (MV_U) of the encoded neighboring upper block to the currentblock, and a second pass of the correction of the prediction image ismade by superimposing the prediction image resulting from the first passand Pred_U. The result of the second pass is the final prediction image.

Note that the above example is of a two-pass correction method using theneighboring left and upper blocks, but the method may be a three-pass orhigher correction method that also uses the neighboring right and/orlower block.

Note that the region subject to superimposition may be the entire pixelregion of the block, and, alternatively, may be a partial block boundaryregion.

Note that here, the prediction image correction process is described asbeing based on a single reference picture, but the same applies when aprediction image is corrected based on a plurality of referencepictures. In such a case, after corrected prediction images resultingfrom performing correction based on each of the reference pictures areobtained, the obtained corrected prediction images are furthersuperimposed to obtain the final prediction image.

Note that the unit of the current block may be a prediction block and,alternatively, may be a sub-block obtained by further dividing theprediction block.

One example of a method for determining whether to implement OBMCprocessing is by using an obmc_flag, which is a signal that indicateswhether to implement OBMC processing. As one specific example, theencoder determines whether the current block belongs to a regionincluding complicated motion. The encoder sets the obmc_flag to a valueof “1” when the block belongs to a region including complicated motionand implements OBMC processing when encoding, and sets the obmc_flag toa value of “0” when the block does not belong to a region includingcomplication motion and encodes without implementing OBMC processing.The decoder switches between implementing OBMC processing or not bydecoding the obmc_flag written in the stream and performing the decodingin accordance with the flag value.

Note that the motion information may be derived on the decoder sidewithout being signalled. For example, a merge mode defined in theH.265/HEVC standard may be used. Moreover, for example, the motioninformation may be derived by performing motion estimation on thedecoder side. In this case, motion estimation is performed without usingthe pixel values of the current block.

Here, a mode for performing motion estimation on the decoder side willbe described. A mode for performing motion estimation on the decoderside is also referred to as pattern matched motion vector derivation(PMMVD) mode or frame rate up-conversion (FRUC) mode.

One example of FRUC processing is illustrated in FIG. 5D. First, acandidate list (a candidate list may be a merge list) of candidates eachincluding a motion vector predictor is generated with reference tomotion vectors of encoded blocks that spatially or temporally neighborthe current block. Next, the best candidate MV is selected from among aplurality of candidate MVs registered in the candidate list. Forexample, evaluation values for the candidates included in the candidatelist are calculated and one candidate is selected based on thecalculated evaluation values.

Next, a motion vector for the current block is derived from the motionvector of the selected candidate. More specifically, for example, themotion vector for the current block is calculated as the motion vectorof the selected candidate (best candidate MV), as-is. Alternatively, themotion vector for the current block may be derived by pattern matchingperformed in the vicinity of a position in a reference picturecorresponding to the motion vector of the selected candidate. In otherwords, when the vicinity of the best candidate MV is searched via thesame method and an MV having a better evaluation value is found, thebest candidate MV may be updated to the MV having the better evaluationvalue, and the MV having the better evaluation value may be used as thefinal MV for the current block. Note that a configuration in which thisprocessing is not implemented is also acceptable.

The same processes may be performed in cases in which the processing isperformed in units of sub-blocks.

Note that an evaluation value is calculated by calculating thedifference in the reconstructed image by pattern matching performedbetween a region in a reference picture corresponding to a motion vectorand a predetermined region. Note that the evaluation value may becalculated by using some other information in addition to thedifference.

The pattern matching used is either first pattern matching or secondpattern matching. First pattern matching and second pattern matching arealso referred to as bilateral matching and template matching,respectively.

In the first pattern matching, pattern matching is performed between twoblocks along the motion trajectory of the current block in two differentreference pictures. Therefore, in the first pattern matching, a regionin another reference picture conforming to the motion trajectory of thecurrent block is used as the predetermined region for theabove-described calculation of the candidate evaluation value.

FIG. 6 is for illustrating one example of pattern matching (bilateralmatching) between two blocks along a motion trajectory. As illustratedin FIG. 6, in the first pattern matching, two motion vectors (MV0, MV1)are derived by finding the best match between two blocks along themotion trajectory of the current block (Cur block) in two differentreference pictures (Ref0, Ref1). More specifically, a difference between(i) a reconstructed image in a specified position in a first encodedreference picture (Ref0) specified by a candidate MV and (ii) areconstructed picture in a specified position in a second encodedreference picture (Ref1) specified by a symmetrical MV scaled at adisplay time interval of the candidate MV may be derived, and theevaluation value for the current block may be calculated by using thederived difference. The candidate MV having the best evaluation valueamong the plurality of candidate MVs may be selected as the final MV.

Under the assumption of continuous motion trajectory, the motion vectors(MV0, MV1) pointing to the two reference blocks shall be proportional tothe temporal distances (TD0, TD1) between the current picture (Cur Pic)and the two reference pictures (Ref0, Ref1). For example, when thecurrent picture is temporally between the two reference pictures and thetemporal distance from the current picture to the two reference picturesis the same, the first pattern matching derives a mirror basedbi-directional motion vector.

In the second pattern matching, pattern matching is performed between atemplate in the current picture (blocks neighboring the current block inthe current picture (for example, the top and/or left neighboringblocks)) and a block in a reference picture. Therefore, in the secondpattern matching, a block neighboring the current block in the currentpicture is used as the predetermined region for the above-describedcalculation of the candidate evaluation value.

FIG. 7 is for illustrating one example of pattern matching (templatematching) between a template in the current picture and a block in areference picture. As illustrated in FIG. 7, in the second patternmatching, a motion vector of the current block is derived by searching areference picture (Ref0) to find the block that best matches neighboringblocks of the current block (Cur block) in the current picture (CurPic). More specifically, a difference between (i) a reconstructed imageof an encoded region that is both or one of the neighboring left andneighboring upper region and (ii) a reconstructed picture in the sameposition in an encoded reference picture (Ref0) specified by a candidateMV may be derived, and the evaluation value for the current block may becalculated by using the derived difference. The candidate MV having thebest evaluation value among the plurality of candidate MVs may beselected as the best candidate MV.

Information indicating whether to apply the FRUC mode or not (referredto as, for example, a FRUC flag) is signalled at the CU level. Moreover,when the FRUC mode is applied (for example, when the FRUC flag is set totrue), information indicating the pattern matching method (first patternmatching or second pattern matching) is signalled at the CU level. Notethat the signaling of such information need not be performed at the CUlevel, and may be performed at another level (for example, at thesequence level, picture level, slice level, tile level, CTU level, orsub-block level).

Here, a mode for deriving a motion vector based on a model assuminguniform linear motion will be described. This mode is also referred toas a bi-directional optical flow (BIO) mode.

FIG. 8 is for illustrating a model assuming uniform linear motion. InFIG. 8, (v_(x), v_(y)) denotes a velocity vector, and τ₀ and τ₁ denotetemporal distances between the current picture (Cur Pic) and tworeference pictures (Ref₀, Ref₁). (MVx₀, MVy₀) denotes a motion vectorcorresponding to reference picture Ref₀, and (MVx₁, MVy₁) denotes amotion vector corresponding to reference picture Ref1.

Here, under the assumption of uniform linear motion exhibited byvelocity vector (v_(x), v_(y)), (MV_(x0), MV_(y0)) and (MVx₁, MVy₁) arerepresented as (v_(x)τ₀, v_(y)τ₀) and (−v_(x)τ₁, −v_(y)τ₁),respectively, and the following optical flow equation is given.

MATH. 1

∂I ^((k)) /∂t+v _(k) ∂I ^((k)) /∂x+v _(y) ∂I ^((k)) /∂y=0.  (1)

Here, I^((k)) denotes a luma value from reference picture k (k=0, 1)after motion compensation. This optical flow equation shows that the sumof (i) the time derivative of the luma value, (ii) the product of thehorizontal velocity and the horizontal component of the spatial gradientof a reference picture, and (iii) the product of the vertical velocityand the vertical component of the spatial gradient of a referencepicture is equal to zero. A motion vector of each block obtained from,for example, a merge list is corrected pixel by pixel based on acombination of the optical flow equation and Hermite interpolation.

Note that a motion vector may be derived on the decoder side using amethod other than deriving a motion vector based on a model assuminguniform linear motion. For example, a motion vector may be derived foreach sub-block based on motion vectors of neighboring blocks.

Here, a mode in which a motion vector is derived for each sub-blockbased on motion vectors of neighboring blocks will be described. Thismode is also referred to as affine motion compensation prediction mode.

FIG. 9A is for illustrating deriving a motion vector of each sub-blockbased on motion vectors of neighboring blocks. In FIG. 9A, the currentblock includes 16 4×4 sub-blocks. Here, motion vector v₀ of the top leftcorner control point in the current block is derived based on motionvectors of neighboring sub-blocks, and motion vector v₁ of the top rightcorner control point in the current block is derived based on motionvectors of neighboring blocks. Then, using the two motion vectors v₀ andv₁, the motion vector (v_(x), v_(y)) of each sub-block in the currentblock is derived using Equation 2 below.

$\begin{matrix}{{MATH}.\mspace{14mu} 2} & \; \\\{ \begin{matrix}{v_{x} = {{\frac{( {v_{1x} - v_{0x}} )}{w}x} - {\frac{( {v_{1y} - v_{0y}} )}{w}y} + v_{0x}}} \\{v_{y} = {{\frac{( {v_{1y} - v_{0y}} )}{w}x} + {\frac{( {v_{1x} - v_{0x}} )}{w}y} + v_{0y}}}\end{matrix}  & (2)\end{matrix}$

Here, x and y are the horizontal and vertical positions of thesub-block, respectively, and w is a predetermined weighted coefficient.

Such an affine motion compensation prediction mode may include a numberof modes of different methods of deriving the motion vectors of the topleft and top right corner control points. Information indicating such anaffine motion compensation prediction mode (referred to as, for example,an affine flag) is signalled at the CU level. Note that the signaling ofinformation indicating the affine motion compensation prediction modeneed not be performed at the CU level, and may be performed at anotherlevel (for example, at the sequence level, picture level, slice level,tile level, CTU level, or sub-block level).

[Prediction Controller]

Prediction controller 128 selects either the intra prediction signal orthe inter prediction signal, and outputs the selected prediction signalto subtractor 104 and adder 116.

Here, an example of deriving a motion vector via merge mode in a currentpicture will be given. FIG. 9B is for illustrating an outline of aprocess for deriving a motion vector via merge mode.

First, an MV predictor list in which candidate MV predictors areregistered is generated. Examples of candidate MV predictors include:spatially neighboring MV predictors, which are MVs of encoded blockspositioned in the spatial vicinity of the current block; a temporallyneighboring MV predictor, which is an MV of a block in an encodedreference picture that neighbors a block in the same location as thecurrent block; a combined MV predictor, which is an MV generated bycombining the MV values of the spatially neighboring MV predictor andthe temporally neighboring MV predictor; and a zero MV predictor, whichis an MV whose value is zero.

Next, the MV of the current block is determined by selecting one MVpredictor from among the plurality of MV predictors registered in the MVpredictor list.

Furthermore, in the variable-length encoder, a merge_idx, which is asignal indicating which MV predictor is selected, is written and encodedinto the stream.

Note that the MV predictors registered in the MV predictor listillustrated in FIG. 9B constitute one example. The number of MVpredictors registered in the MV predictor list may be different from thenumber illustrated in FIG. 9B, the MV predictors registered in the MVpredictor list may omit one or more of the types of MV predictors givenin the example in FIG. 9B, and the MV predictors registered in the MVpredictor list may include one or more types of MV predictors inaddition to and different from the types given in the example in FIG.9B.

Note that the final MV may be determined by performing DMVR processing(to be described later) by using the MV of the current block derived viamerge mode.

Here, an example of determining an MV by using DMVR processing will begiven.

FIG. 9C is a conceptual diagram for illustrating an outline of DMVRprocessing.

First, the most appropriate MVP set for the current block is consideredto be the candidate MV, reference pixels are obtained from a firstreference picture, which is a picture processed in the L0 direction inaccordance with the candidate MV, and a second reference picture, whichis a picture processed in the L1 direction in accordance with thecandidate MV, and a template is generated by calculating the average ofthe reference pixels.

Next, using the template, the surrounding regions of the candidate MVsof the first and second reference pictures are searched, and the MV withthe lowest cost is determined to be the final MV. Note that the costvalue is calculated using, for example, the difference between eachpixel value in the template and each pixel value in the regionssearched, as well as the MV value.

Note that the outlines of the processes described here are fundamentallythe same in both the encoder and the decoder.

Note that processing other than the processing exactly as describedabove may be used, so long as the processing is capable of deriving thefinal MV by searching the surroundings of the candidate MV.

Here, an example of a mode that generates a prediction image by usingLIC processing will be given.

FIG. 9D is for illustrating an outline of a prediction image generationmethod using a luminance correction process performed via LICprocessing.

First, an MV is extracted for obtaining, from an encoded referencepicture, a reference image corresponding to the current block.

Next, information indicating how the luminance value changed between thereference picture and the current picture is extracted and a luminancecorrection parameter is calculated by using the luminance pixel valuesfor the encoded left neighboring reference region and the encoded upperneighboring reference region, and the luminance pixel value in the samelocation in the reference picture specified by the MV.

The prediction image for the current block is generated by performing aluminance correction process by using the luminance correction parameteron the reference image in the reference picture specified by the MV.

Note that the shape of the surrounding reference region illustrated inFIG. 9D is just one example; the surrounding reference region may have adifferent shape.

Moreover, although a prediction image is generated from a singlereference picture in this example, in cases in which a prediction imageis generated from a plurality of reference pictures as well, theprediction image is generated after performing a luminance correctionprocess, via the same method, on the reference images obtained from thereference pictures.

One example of a method for determining whether to implement LICprocessing is by using an lic_flag, which is a signal that indicateswhether to implement LIC processing. As one specific example, theencoder determines whether the current block belongs to a region ofluminance change. The encoder sets the lic_flag to a value of “1” whenthe block belongs to a region of luminance change and implements LICprocessing when encoding, and sets the lic_flag to a value of “0” whenthe block does not belong to a region of luminance change and encodeswithout implementing LIC processing. The decoder switches betweenimplementing LIC processing or not by decoding the lic_flag written inthe stream and performing the decoding in accordance with the flagvalue.

One example of a different method of determining whether to implementLIC processing is determining so in accordance with whether IACprocessing was determined to be implemented for a surrounding block. Inone specific example, when merge mode is used on the current block,whether LIC processing was applied in the encoding of the surroundingencoded block selected upon deriving the MV in the merge mode processingmay be determined, and whether to implement LIC processing or not can beswitched based on the result of the determination. Note that in thisexample, the same applies to the processing performed on the decoderside.

[Decoder Outline]

Next, a decoder capable of decoding an encoded signal (encodedbitstream) output from encoder 100 will be described. FIG. 10 is a blockdiagram illustrating a functional configuration of decoder 200 accordingto Embodiment 1. Decoder 200 is a moving picture/picture decoder thatdecodes a moving picture/picture block by block.

As illustrated in FIG. 10, decoder 200 includes entropy decoder 202,inverse quantizer 204, inverse transformer 206, adder 208, block memory210, loop filter 212, frame memory 214, intra predictor 216, interpredictor 218, and prediction controller 220.

Decoder 200 is realized as, for example, a generic processor and memory.In this case, when a software program stored in the memory is executedby the processor, the processor functions as entropy decoder 202,inverse quantizer 204, inverse transformer 206, adder 208, loop filter212, intra predictor 216, inter predictor 218, and prediction controller220. Alternatively, decoder 200 may be realized as one or more dedicatedelectronic circuits corresponding to entropy decoder 202, inversequantizer 204, inverse transformer 206, adder 208, loop filter 212,intra predictor 216, inter predictor 218, and prediction controller 220.

Hereinafter, each component included in decoder 200 will be described.

[Entropy Decoder]

Entropy decoder 202 entropy decodes an encoded bitstream. Morespecifically, for example, entropy decoder 202 arithmetic decodes anencoded bitstream into a binary signal. Entropy decoder 202 thendebinarizes the binary signal. With this, entropy decoder 202 outputsquantized coefficients of each block to inverse quantizer 204.

[Inverse Quantizer]

Inverse quantizer 204 inverse quantizes quantized coefficients of ablock to be decoded (hereinafter referred to as a current block), whichare inputs from entropy decoder 202. More specifically, inversequantizer 204 inverse quantizes quantized coefficients of the currentblock based on quantization parameters corresponding to the quantizedcoefficients. Inverse quantizer 204 then outputs the inverse quantizedcoefficients (i.e., transform coefficients) of the current block toinverse transformer 206.

[Inverse Transformer]

Inverse transformer 206 restores prediction errors by inversetransforming transform coefficients, which are inputs from inversequantizer 204.

For example, when information parsed from an encoded bitstream indicatesapplication of EMT or AMT (for example, when the AMT flag is set totrue), inverse transformer 206 inverse transforms the transformcoefficients of the current block based on information indicating theparsed transform type.

Moreover, for example, when information parsed from an encoded bitstreamindicates application of NSST, inverse transformer 206 applies asecondary inverse transform to the transform coefficients.

[Adder]

Adder 208 reconstructs the current block by summing prediction errors,which are inputs from inverse transformer 206, and prediction samples,which is an input from prediction controller 220. Adder 208 then outputsthe reconstructed block to block memory 210 and loop filter 212.

[Block Memory]

Block memory 210 is storage for storing blocks in a picture to bedecoded (hereinafter referred to as a current picture) for reference inintra prediction. More specifically, block memory 210 storesreconstructed blocks output from adder 208.

[Loop Filter]

Loop filter 212 applies a loop filter to blocks reconstructed by adder208, and outputs the filtered reconstructed blocks to frame memory 214and, for example, a display device.

When information indicating the enabling or disabling of ALF parsed froman encoded bitstream indicates enabled, one filter from among aplurality of filters is selected based on direction and activity oflocal gradients, and the selected filter is applied to the reconstructedblock.

[Frame Memory]

Frame memory 214 is storage for storing reference pictures used in interprediction, and is also referred to as a frame buffer. Morespecifically, frame memory 214 stores reconstructed blocks filtered byloop filter 212.

[Intra Predictor]

Intra predictor 216 generates a prediction signal (intra predictionsignal) by intra prediction with reference to a block or blocks in thecurrent picture and stored in block memory 210. More specifically, intrapredictor 216 generates an intra prediction signal by intra predictionwith reference to samples (for example, luma and/or chroma values) of ablock or blocks neighboring the current block, and then outputs theintra prediction signal to prediction controller 220.

Note that when an intra prediction mode in which a chroma block is intrapredicted from a luma block is selected, intra predictor 216 may predictthe chroma component of the current block based on the luma component ofthe current block.

Moreover, when information indicating the application of PDPC is parsedfrom an encoded bitstream, intra predictor 216 correctspost-intra-prediction pixel values based on horizontal/verticalreference pixel gradients.

[Inter Predictor]

Inter predictor 218 predicts the current block with reference to areference picture stored in frame memory 214. Inter prediction isperformed per current block or per sub-block (for example, per 4×4block) in the current block. For example, inter predictor 218 generatesan inter prediction signal of the current block or sub-block by motioncompensation by using motion information (for example, a motion vector)parsed from an encoded bitstream, and outputs the inter predictionsignal to prediction controller 220.

Note that when the information parsed from the encoded bitstreamindicates application of OBMC mode, inter predictor 218 generates theinter prediction signal using motion information for a neighboring blockin addition to motion information for the current block obtained frommotion estimation.

Moreover, when the information parsed from the encoded bitstreamindicates application of FRUC mode, inter predictor 218 derives motioninformation by performing motion estimation in accordance with thepattern matching method (bilateral matching or template matching) parsedfrom the encoded bitstream. Inter predictor 218 then performs motioncompensation using the derived motion information.

Moreover, when BIO mode is to be applied, inter predictor 218 derives amotion vector based on a model assuming uniform linear motion. Moreover,when the information parsed from the encoded bitstream indicates thataffine motion compensation prediction mode is to be applied, interpredictor 218 derives a motion vector of each sub-block based on motionvectors of neighboring blocks.

[Prediction Controller]

Prediction controller 220 selects either the intra prediction signal orthe inter prediction signal, and outputs the selected prediction signalto adder 208.

Comparative Examples

Prior to describing inter frame prediction according to the presentembodiment, examples of inter frame prediction when methods of thepresent embodiment are not used will be described.

First, Comparative Example 1 will be described. FIG. 11 is a flow chartillustrating inter frame prediction in a unit of a prediction block in avideo encoding method and a video decoding method according toComparative Example 1. The processing illustrated in FIG. 11 is repeatedin the unit of the prediction block that is a processing unit for interframe prediction. It should be noted that although the following mainlydescribes how inter predictor 126 of encoder 100 operates, interpredictor 218 of decoder 200 operates in the same manner.

When FRUC control information indicates 0 (0 in S101), inter predictor126 derives a motion vector (MV) in the unit of the prediction blockaccording to a normal inter prediction mode (S102). Here, the normalinter frame prediction mode is a conventional mode using no FRUC mode,and is, for example, a mode that derives a motion vector in an encodingside, and transmits, from the encoding side to a decoding side,information indicating the derived motion vector.

Next, inter predictor 126 obtains an inter frame prediction image byperforming motion compensation in the unit of the prediction block usingthe motion vector in the unit of the prediction block (S103).

In contrast, when the FRUC control information indicates 1 (1 in S101),inter predictor 126 derives a motion vector in the unit of theprediction block according to a template FRUC mode (S104). After that,inter predictor 126 derives a motion vector in a unit of a sub-blockobtained by splitting a prediction block, according to the template FRUCmode (S105).

On the other hand, when the FRUC control information indicates 2 (2 inS101), inter predictor 126 derives a motion vector in the unit of theprediction block according to a bilateral FRUC mode (S106). After that,inter predictor 126 derives a motion vector in the unit of the sub-blockobtained by splitting the prediction block, according to the bilateralFRUC mode (S107).

After inter predictor 126 derives the motion vector in the unit of thesub-block according to the template FRUC mode or the bilateral FRUCmode, inter predictor 126 obtains an inter frame prediction image byperforming motion compensation in the unit of the sub-block using thederived motion vector in the unit of the sub-block (S108).

As seen above, FRUC makes it possible to track a minute motion bycorrecting a motion vector in the unit of the sub-block after deriving amotion vector in a unit of a block. As a result, it is possible toimprove coding efficiency. At the same time, there is a possibility thata block in which a change in shape occurs in a unit of a pixel is notsufficiently addressed.

Next, Comparative Example 2 will be described. FIG. 12 is a flow chartillustrating inter frame prediction in a unit of a prediction block in avideo encoding method and a video decoding method according toComparative Example 2. In Comparative Example 2, BIO is used as motioncompensation. Stated differently, steps S103 and S108 of the processingillustrated in FIG. 11 are replaced with steps S103A and S108A of theprocessing illustrated in FIG. 12.

When FRUC control information indicates 0 (0 in S101), inter predictor126 derives a motion vector in the unit of the prediction blockaccording to the normal inter prediction mode (S102). Next, interpredictor 126 obtains an inter frame prediction image by performingmotion compensation including BIO in the unit of the prediction blockusing the motion vector in the unit of the prediction block (S103A).

In contrast, when the FRUC control information indicates 1 (1 in S101),inter predictor 126 derives a motion vector in the unit of theprediction block according to the template FRUC mode (S104). After that,inter predictor 126 derives a motion vector in a unit of a sub-blockobtained by splitting a prediction block, according to the template FRUCmode (S105).

On the other hand, when the FRUC control information indicates 2 (2 inS101), inter predictor 126 derives a motion vector in the unit of theprediction block according to the bilateral FRUC mode (S106). Afterthat, inter predictor 126 derives a motion vector in the unit of thesub-block obtained by splitting the prediction block, according to thebilateral FRUC mode (S107).

After inter predictor 126 derives the motion vector in the unit of thesub-block according to the template FRUC mode or the bilateral FRUCmode, inter predictor 126 obtains an inter frame prediction image byperforming motion compensation including BIO in the unit of thesub-block using the derived motion vector in the unit of the sub-block(S108A).

As seen above, in Comparative Example 2, inter predictor 126 can correctthe prediction image in a unit of a pixel by performing BIO after FRUC.In consequence, there is a possibility of improving coding efficiencyfor a block in which a change in shape occurs.

At the same time, there is a problem that an amount of processingincreases because both FRUC and BIO are performed.

Moreover, BIO is performed in the unit of the prediction block in thenormal inter frame prediction mode, and BIO is performed in the unit ofthe sub-block in the FRUC mode. As such, there is also a problem that itbecomes necessary to implement a function for two types of BIO becausethe normal inter frame prediction mode and the FRUC mode differ in aunit of a motion vector to be an input for BIO.

[Inter Frame Prediction]

Inter frame prediction performed by inter predictor 126 according to thepresent embodiment will be described. Inter predictor 126 can execute atleast two types of motion vector derivation modes including the normalinter frame prediction mode and the FRUC mode, in the inter frameprediction. In the normal inter frame prediction mode, informationrelating to a motion vector of a current block is encoded into a stream.In the FRUC mode, information relating to a motion vector of a currentblock is not encoded into a stream, and the motion vector is derivedusing a reconstructed image of a processed region and a processedreference picture by a method common to the encoding side and thedecoding side.

Inter predictor 126 further performs BIO that obtains a prediction imageby performing motion compensation of the processed reference picture foreach prediction block using the motion vector, derives a local motion 16estimation value by obtaining a gradient value of luminance, andgenerates a prediction image corrected by using the derived local motionestimation value. In the FRUC mode, inter predictor 126 performsprocessing in a unit of a prediction block, and derives a motion vectorin the unit of the prediction block. Moreover, in BIO, inter predictor126 always uses a motion vector in the unit of the prediction block asan input when any of the motion vector derivation modes is used, andgenerates a prediction image using common processing in the unit of theprediction block.

Hereinafter, inter frame prediction performed by inter predictor 126according to the present embodiment will be described. FIG. 13 is a flowchart illustrating inter frame prediction in a unit of a predictionblock in a video encoding method and a video decoding method accordingto the present embodiment. The processing illustrated in FIG. 13 isrepeated in the unit of the prediction block that is a processing unitfor inter frame prediction. It should be noted that although thefollowing mainly describes how inter predictor 126 of encoder 100operates, inter predictor 218 of decoder 200 operates in the samemanner.

When FRUC control information indicates 0 (0 in S101), as with theprocessing illustrated in FIG. 12, inter predictor 126 derives a motionvector in the unit of the prediction block according to the normal interprediction mode (S102). Next, inter predictor 126 obtains an inter frameprediction image by performing motion compensation including BIO in theunit of the prediction block using the motion vector in the unit of theprediction block (S103A).

In contrast, when the FRUC control information indicates 1 (1 in S101),inter predictor 126 derives a motion vector in the unit of theprediction block according to the template FRUC mode (S104). On theother hand, when the FRUC control information indicates 2 (2 in S101),inter predictor 126 derives a motion vector in the unit of theprediction block according to the bilateral FRUC mode (S106). It shouldbe noted that unlike the processing illustrated in FIG. 12, in theprocessing illustrated in FIG. 13, inter predictor 126 does not derive amotion vector in a unit of a sub-block when the FRUC mode is used.

After inter predictor 126 derives the motion vector in the unit of theprediction block according to the template FRUC mode or the bilateralFRUC mode, inter predictor 126 obtains an inter frame prediction imageby performing motion compensation including BIO in the unit of theprediction block using the derived motion vector in the unit of theprediction block (S103A).

As seen above, in the present embodiment, even when the FRUC controlinformation indicates any of the normal inter frame prediction mode, thetemplate FRUC mode, and the bilateral FRUC mode, inter predictor 126derives the motion vector in the unit of the prediction block. Further,inter predictor 126 performs BIO in the unit of the prediction block.That is to say, in any of the modes, a processing unit is the unit ofthe prediction block, and an identical processing unit is used.

It should be noted that the numbers used for the FRUC controlinformation above are one example, and numbers other than those may beused. Further, only one of the template FRUC mode and the bilateral FRUCmode may be used. In addition, common processing may be used between theencoding and the decoding.

Here, in Comparative Example 2 illustrated in FIG. 12, the FRUC in theunit of the sub-block that corrects the motion vector in the unit of thesub-block and the BIO that corrects the prediction image in the unit ofthe pixel are both performed. The FRUC in the unit of the sub-block andthe BIO both perform correction in a unit finer than the predictionblock, have the same properties, and produce similar effects. In theprocessing illustrated in FIG. 13, these processes are consolidated intothe BIO. Further, in the processing illustrated in FIG. 13, it ispossible to reduce an amount of processing by not performing the FRUC inthe unit of the sub-block with a large amount of processing. Inaddition, since the unit of the sub-block is switched to the unit of theprediction block for BIO when the FRUC mode is used, it is possible toreduce an amount of processing.

As stated above, the processing according to the present embodimentillustrated in FIG. 13 has a possibility of improving, while suppressingan increase in the amount of processing, coding efficiency for a blockin which a change in shape occurs, compared to Comparative Example 2illustrated in FIG. 12.

Moreover, in the processing according to the present embodimentillustrated in FIG. 13, since BIO using the motion vector in the unit ofthe prediction block as an input is performed when the FRUC controlinformation indicates any of the values, BIO using the motion vector ofthe sub-block as an input becomes unnecessary, compared to ComparativeExample 2 illustrated in FIG. 12. Accordingly, it is possible tosimplify implementation.

It should be noted that BIO in step S103A need not be completelyidentical in motion vector derivation modes. In other words, differentBIO may be used in any or each of the normal inter frame predictionmode, the template FRUC mode, and the bilateral FRUC mode.

Moreover, regarding at least any one of the motion vector derivationmodes, BIO in step S103A may be replaced with another process (includinga modification of BIO) of generating a prediction image while correctingthe prediction image in the unit of the pixel or a unit finer than aprediction block. In this case also, since the above-described twoprocesses having the same properties can be consolidated into oneprocess, there is a possibility of improving, while suppressing anincrease in the amount of processing, coding efficiency for a block inwhich a change in shape occurs, compared to Comparative Example 2.

Furthermore, regarding at least any one of the motion vector derivationmodes, BIO in step S103A may be replaced with another process ofgenerating a prediction image while correcting the prediction imageusing the motion vector in the unit of the prediction block as an input.In this case also, the above-described process using the motion vectorin the unit of the sub-block as an input becomes unnecessary, and it ispossible to simplify implementation.

Moreover, it may be switched whether to perform BIO. For example, interpredictor 126 may perform the processing illustrated in FIG. 13 wheninter predictor 126 performs BIO, and may perform the processingillustrated in FIG. 11 when inter predictor 126 does not perform BIO.Alternatively, as is the case with FIG. 13, inter predictor 126 need notperform the derivation of a motion vector in the unit of the sub-blockeven when inter predictor 126 does not perform BIO. In other words,inter predictor 126 may perform motion compensation in the unit of theprediction block not including BIO instead of step S108A illustrated inFIG. 13.

[Template FRUC Mode and Bilateral FRUC Mode]

Hereinafter, a method of deriving a motion vector according to thetemplate FRUC mode or the bilateral FRUC mode will be described. Amethod of deriving a motion vector in a unit of a block is basicallyidentical to a method of deriving a motion vector in a unit of asub-block. In what follows, a method of deriving a motion vector of ablock and a method of deriving a motion vector of a sub-block are eachdescribed as a method of deriving a motion vector of a current region.

FIG. 14 is a conceptual diagram illustrating the template FRUC mode foruse in deriving a motion vector of a current region in encoder 100 anddecoder 200. In the template FRUC mode, a motion vector of a currentregion is derived using a method common to encoder 100 and decoder 200without encoding and decoding the information of the motion vector.

Further, in the template FRUC mode, a motion vector is derived using areconstructed image of a neighboring region that is a region neighboringa current region, and a reconstructed image of a correspondingneighboring region that is a region in a reference picture.

Here, the neighboring region is one or both of the region adjacent tothe left of the current region and the region adjacent to the top of thecurrent region.

The corresponding neighboring region is a region specified using amotion vector candidate that is a candidate for the motion vector of thecurrent region. Specifically, the corresponding neighboring region is aregion specified from the neighboring region by a motion vectorcandidate. Moreover, the relative position of the correspondingneighboring region with respect to a corresponding region specified fromthe current region by a motion vector candidate is identical to therelative position of the neighboring region with respect to the currentregion.

FIG. 15 is a conceptual diagram illustrating the bilateral FRUC mode foruse in deriving a motion vector of a current region in encoder 100 anddecoder 200. As with the template FRUC mode, in the bilateral FRUC mode,a motion vector of a current region is derived using a method common toencoder 1.00 and decoder 200 without encoding and decoding theinformation of the motion vector.

Moreover, in the bilateral FRUC mode, a motion vector is derived usingtwo reconstructed images of two reference regions in two referencepictures. For example, as illustrated in FIG. 15, a motion vector isderived using the reconstructed image of the corresponding region in thefirst reference picture and the reconstructed image of the symmetricalregion in the second reference picture.

Here, each of the corresponding region and the symmetrical region is aregion specified by a motion vector candidate that is a candidate forthe motion vector of the current region. Specifically, the correspondingregion is a region specified from the current region by the motionvector candidate. The symmetrical region is a region specified from thecurrent region by a symmetrical motion vector. The symmetrical motionvector is a motion vector paired with a motion vector candidate forbi-directional prediction. The symmetrical motion vector may be a motionvector derived by scaling a motion vector candidate.

FIG. 16 is a flow chart illustrating an operation of deriving a motionvector by inter predictor 126 of encoder 100 according to the templateFRUC mode or the bilateral FRUC mode. Inter predictor 21.8 of decoder200 operates in the same manner as inter predictor 126 of encoder 1.00.

First, inter predictor 126 derives a motion vector candidate byreference to a motion vector of each of one or more processed regionstemporally or spatially surrounding a current region.

In the bilateral FRUC mode, inter predictor 126 derives a motion vectorcandidate for bi-directional prediction. In other words, inter predictor126 derives the motion vector candidate as a set of two motion vectors.

Specifically, in the bilateral FRUC mode, when a motion vector of aprocessed region is a motion vector for bi-directional prediction, interpredictor 126 derives the motion vector for bi-directional predictiondirectly as the motion vector candidate for bi-directional prediction.When the motion vector of the processed region is a motion vector forunidirectional prediction, inter predictor 126 may derive the motionvector candidate for bi-directional prediction by deriving a motionvector for bi-directional prediction by, for example, scaling the motionvector for unidirectional prediction.

More specifically, in the bilateral FRUC mode, inter predictor 126derives a motion vector referring to the second reference picture byscaling a motion vector referring to the first reference picture atdisplay time intervals. Accordingly, inter predictor 126 derives, as themotion vector candidate for bi-directional prediction, a motion vectorcandidate including the motion vector for unidirectional predictionpaired with the scaled motion vector.

Alternatively, in the bilateral FRUC mode, when the motion vector of theprocessed region is the motion vector for bi-directional prediction,inter predictor 126 may derive the motion vector of the processed regionas a motion vector candidate. When the motion vector of the processedregion is the motion vector for unidirectional prediction, interpredictor 126 need not derive the motion vector of the processed regionas the motion vector candidate.

In the template FRUC mode, inter predictor 126 derives a motion vectorof a processed region as a motion vector candidate regardless of whetherthe motion vector of the processed region is a motion vector forbi-directional prediction or a motion vector for unidirectionalprediction.

Inter predictor 126 generates a motion vector candidate list includingmotion vector candidates (8201). Here, when the current region is asub-block, that is, a motion vector is derived in a unit of thesub-block, inter predictor 126 may include, as a motion vectorcandidate, a motion vector in a unit of a block in the motion vectorcandidate list. At that time, inter predictor 126 may include, as themost preferential motion vector candidate, the motion vector in the unitof the block in the motion vector candidate list.

Moreover, in the bilateral FRUC mode, when the motion vector in the unitof the block is a motion vector for unidirectional prediction, interpredictor 126 may derive a motion vector candidate for bi-directionalprediction from the motion vector for unidirectional prediction byscaling etc. For example, as with a case in which a surrounding motionvector is the motion vector for unidirectional prediction, interpredictor 126 may derive the motion vector candidate for bi-directionalprediction from the motion vector for unidirectional prediction byscaling etc.

Inter predictor 126 may include a motion vector candidate derived as themotion vector candidate for bi-directional prediction from the motionvector for unidirectional prediction, in the motion vector candidatelist.

Alternatively, in the bilateral FRUC mode, when the motion vector in theunit of the block is the motion vector for bi-directional prediction,inter predictor 126 may include the motion vector in the unit of theblock as a motion vector candidate in the motion vector candidate list.In addition, when the motion vector in the unit of the block is themotion vector for unidirectional prediction, inter predictor 126 neednot include the motion vector in the unit of the block as a motionvector candidate in the motion vector candidate list.

Next, inter predictor 126 selects the best motion vector candidate fromamong one or more motion vector candidates included in the motion vectorcandidate list (S202). At that time, inter predictor 126 calculates anevaluation value for each of the one or more motion vector candidatesaccording to a degree of matching between two reconstructed images oftwo evaluation-target regions.

Specifically, in the template FRUC mode, the two evaluation-targetregions are a neighboring region and a corresponding neighboring regionas illustrated in FIG. 14, and in the bilateral FRUC mode, the twoevaluation-target regions are a corresponding region and a symmetricalregion as illustrated in FIG. 15. As stated above, the correspondingneighboring region used in the template FRUC mode as well as thecorresponding region and the symmetrical region used in the bilateralFRUC mode are determined according to the motion vector candidate.

For example, inter predictor 126 calculates a higher evaluation value asa degree of matching between the two reconstructed images of the twoevaluation-target regions is higher. Specifically, inter predictor 126derives a difference value of the two reconstructed images of the twoevaluation-target regions. Then, inter predictor 126 calculates anevaluation value using the difference value. For example, interpredictor 126 calculates a higher evaluation value as a difference valueis lower.

Moreover, not only a difference value but also other information may beused in calculating an evaluation value. In other words, inter predictor126 may calculate an evaluation value using a difference value and otherinformation. For example, the priority of one or more motion vectorcandidates, a coding amount based on the priority, etc. may affect anevaluation value.

Next, inter predictor 126 selects, as the best motion vector candidate,a motion vector candidate having the highest evaluation value from amongthe one or more motion vector candidates.

Next, inter predictor 126 derives a motion vector of a current region bysearching the vicinity of the best motion vector candidate (S203).

In other words, inter predictor 1.26 similarly calculates an evaluationvalue for a motion vector indicating a region surrounding the regionindicated by the best motion vector candidate. When there is a motionvector having an evaluation value higher than that of the best motionvector candidate, inter predictor 126 updates the best motion vectorcandidate with the motion vector having the evaluation value higher thanthat of the best motion vector candidate. Then, inter predictor 126derives the updated best motion vector candidate as the final motionvector of the current region.

It should be noted that inter predictor 126 may derive the best motionvector candidate as the final motion vector of the current regionwithout searching the vicinity of the best motion vector candidate(S203). Moreover, the best motion vector candidate is not limited to amotion vector candidate having the highest evaluation value. One of oneor more motion vector candidates having at least standard evaluationvalues may be selected as the best motion vector candidate according toa predetermined order of priority.

Furthermore, here, a process pertaining to the current region and theprocessed region is encoding or decoding, for example. Morespecifically, the process pertaining to the current region and theprocessed region may be deriving a motion vector. Alternatively, theprocess pertaining to the current region and the processed region may bereconstructing.

[BIO]

FIG. 17 is a conceptual diagram illustrating BIO in encoder 100 anddecoder 200. In BIO, a prediction image of a current block is generatedby reference to a spatial gradient of luminance in an image obtained byperforming motion compensation of the current block using a motionvector of the current block.

An L0 motion vector (MV-L0) and an L1 motion vector (MV_L1) that are twomotion vectors of the current block are derived prior to BIO. The L0motion vector (MV_L0) is a motion vector for referring to an L0reference picture that is a processed picture, and the L1 motion vector(MV_L1) is a motion vector for referring to an L1 reference picture thatis a processed picture. The L0 reference picture and the L1 referencepicture are two reference pictures simultaneously referred to inbi-prediction of the current block.

A normal inter frame prediction mode, a merge mode, a FRUC mode, etc.may be used as a method for deriving the L0 motion vector (MV_L0) andthe L1 motion vector (MV_L1). For example, in the normal inter frameprediction mode, encoder 100 derives a motion vector by performingmotion detection using an image of a current block, and encodesinformation of the motion vector. Further, in the normal inter frameprediction mode, decoder 200 derives the motion vector by decoding theinformation of the motion vector.

Then, in BIO, an L0 prediction image is obtained by referring to the L0reference picture and performing motion compensation of the currentblock using the L0 motion vector (MV_L0). For example, the L0 predictionimage may be obtained by applying a motion compensation filter to animage of an L0 reference pixel range including a block specified fromthe current block in the L0 reference picture by the L1 motion vector(MV_L0), and the surroundings of the block.

Moreover, an L0 gradient image is obtained that indicates a spatialgradient of luminance in each pixel of the L0 prediction image. Forexample, the L0 gradient image is obtained by referring to the luminanceof each pixel in the L0 reference pixel range including the blockspecified from the current block in the L0 reference picture by the L0motion vector (MV_L0), and the surroundings of the block.

Moreover, an L1 prediction image is obtained by referring to the L1reference picture and performing motion compensation of the currentblock using the L1 motion vector (MV_L1). For example, the L1 predictionimage may be obtained by applying a motion compensation filter to animage of an L1 reference pixel range including a block specified fromthe current block in the L1 reference picture by the L1 motion vector(MV_L1), and the surroundings of the block.

Moreover, an L1 gradient image is obtained that indicates a spatialgradient of luminance in each pixel of the L1 prediction image. Forexample, the L1 gradient image is obtained by referring to the luminanceof each pixel in the L1 reference pixel range including the blockspecified from the current block in the L1 reference picture by the L1motion vector (MV_L1), and the surroundings of the block.

Then, a local motion estimation value is derived for each pixel of thecurrent block. Specifically, at that time, a pixel value of acorresponding pixel position in the L0 prediction image, a gradientvalue of a corresponding pixel position in the L0 gradient image, apixel value of a corresponding pixel position in the L1 predictionimage, and a gradient value of a corresponding pixel position in the L1gradient value are used. The local motion estimation value can be alsoreferred to as a correction motion vector (correction MV).

Then, a pixel correction value is derived for each pixel of the currentblock using the gradient value of the corresponding pixel position inthe L0 gradient image, the gradient value of the corresponding pixelposition in the L1 gradient image, and the local motion estimationvalue. After that, a prediction pixel value is derived for each pixel ofthe current block using the pixel value of the corresponding pixelposition in the L0 prediction image, the pixel value of thecorresponding pixel position in the L1 prediction image, and the pixelcorrection value. With this, a prediction image to which BIO is appliedis derived.

In other words, the prediction pixel value obtained using the pixelvalue of the corresponding pixel position in the L0 prediction image andthe pixel value of the corresponding pixel position in the L1 predictionimage is corrected with the pixel correction value. To further put itdifferently, the prediction image obtained using the L0 prediction imageand the L1 prediction image is corrected using the spatial gradients ofthe luminance in the L0 prediction image and the L1 prediction image.

FIG. 18 is a flow chart illustrating operations performed as BIO byinter predictor 126 of encoder 100. Inter predictor 218 of decoder 200operates in the same manner as inter predictor 126 of encoder 100.

First, inter predictor 126 obtains an L0 prediction image by referenceto an L0 reference picture using an L0 motion vector (MV_L0) (S401).Then, inter predictor 126 obtains an L0 gradient image by reference tothe L0 reference picture using the L0 motion vector (8402).

Similarly, inter predictor 126 obtains an L1 prediction image byreference to an L1 reference picture using an L1 motion vector (MV_L1)(S401). Then, inter predictor 126 obtains an L1 gradient image byreference to the L1 reference picture using the L1 motion vector (S402).

Next, inter predictor 126 derives a local motion estimation value foreach pixel of a current block (S411). At that time, a pixel value of acorresponding pixel position in the L0 prediction image, a gradientvalue of a corresponding pixel position in the L0 gradient image, apixel value of a corresponding pixel position in the L1 predictionimage, and a gradient value of a corresponding pixel position in the L1gradient value are used.

Then, inter predictor 126 derives a pixel correction value for eachpixel of the current block using the gradient value of the correspondingpixel position in the L0 gradient image, the gradient value of thecorresponding pixel position in the L1 gradient image, and the localmotion estimation value. After that, inter predictor 126 derives aprediction pixel value for each pixel of the current block using thepixel value of the corresponding pixel position in the L0 predictionimage, the pixel value of the corresponding pixel position in the L1prediction image, and the pixel correction value (S412).

Inter predictor 126 generates, by the above operations, a predictionimage to which BIO is applied

It should be noted that specifically, the following equation (3) may beused in deriving a local motion estimation value and a pixel correctionvalue.

$\begin{matrix}{\mspace{79mu}{{MATH}.\mspace{14mu} 3}} & \; \\{\mspace{79mu}{{{G_{x}\lbrack {x,y} \rbrack} = {{I_{x}^{0}\lbrack {x,y} \rbrack} + {I_{x}^{1}\lbrack {x,y} \rbrack}}}\mspace{79mu}{{G_{y}\lbrack {x,y} \rbrack} = {{I_{y}^{0}\lbrack {x,y} \rbrack} + {I_{y}^{1}\lbrack {x,y} \rbrack}}}\mspace{79mu}{{\Delta\;{I\lbrack {x,y} \rbrack}} = {{I^{0}\lbrack {x,y} \rbrack} - {I^{1}\lbrack {x,y} \rbrack}}}\mspace{79mu}{{G_{x}{G_{y}\lbrack {x,y} \rbrack}} = {{G_{x}\lbrack {x,y} \rbrack}*{G_{y}\lbrack {x,y} \rbrack}}}\mspace{79mu}{{{sG}_{x}{G_{y}\lbrack {x,y} \rbrack}} = {\sum_{{\lbrack{i,j}\rbrack} \in \Omega}{{w\lbrack {i,j} \rbrack}*G_{x}{G_{y}\lbrack {i,j} \rbrack}}}}\mspace{79mu}{{{sG}_{x}^{2}\lbrack {x,y} \rbrack} = {\sum_{{\lbrack{i,j}\rbrack} \in \Omega}{{w\lbrack {i,j} \rbrack}*{G_{x}\lbrack {i,j} \rbrack}*{G_{x}\lbrack {i,j} \rbrack}}}}\mspace{79mu}{{{sG}_{y}^{2}\lbrack {x,y} \rbrack} = {\sum_{{\lbrack{i,j}\rbrack} \in \Omega}{{w\lbrack {i,j} \rbrack}*{G_{y}\lbrack {i,j} \rbrack}*{G_{y}\lbrack {i,j} \rbrack}}}}\mspace{79mu}{{{sG}_{x}{{dI}\lbrack {x,y} \rbrack}} = {\sum_{{\lbrack{i,j}\rbrack} \in \Omega}{{w\lbrack {i,j} \rbrack}*{G_{x}\lbrack {i,j} \rbrack}*\Delta\;{I\lbrack {i,j} \rbrack}}}}\mspace{79mu}{{{sG}_{y}{{dI}\lbrack {x,y} \rbrack}} = {\sum_{{\lbrack{i,j}\rbrack} \in \Omega}{{w\lbrack {i,j} \rbrack}*{G_{y}\lbrack {i,j} \rbrack}*\Delta\;{I\lbrack {i,j} \rbrack}}}}\mspace{79mu}{{u\lbrack {x,y} \rbrack} = {{sG}_{x}{{{dI}\lbrack {x,y} \rbrack}/{{sG}_{x}^{2}\lbrack {x,y} \rbrack}}}}\mspace{79mu}{{v\lbrack {x,y} \rbrack} = {( {{{sG}_{y}{{dI}\lbrack {x,y} \rbrack}} - {{u\lbrack {x,y} \rbrack}*{sG}_{x}{G_{y}\lbrack {x,y} \rbrack}}} )/{{sG}_{y}^{2}\lbrack {x,y} \rbrack}}}{{b\lbrack {x,y} \rbrack} = {{{u\lbrack {x,y} \rbrack}*( {{I_{x}^{0}\lbrack {x,y} \rbrack} - {I_{x}^{1}\lbrack {x,y} \rbrack}} )} + {{v\lbrack {x,y} \rbrack}*( {{I_{y}^{0}\lbrack {x,y} \rbrack} - {I_{y}^{1}\lbrack {x,y} \rbrack}} )}}}\mspace{79mu}{p = {( {{I^{0}\lbrack {x,y} \rbrack} + {I^{1}\lbrack {x,y} \rbrack} + {b\lbrack {x,y} \rbrack}} ) ⪢ 1}}}} & (3)\end{matrix}$

In equation (3), I_(x) ⁰[x, y] denotes a horizontal gradient value of apixel position [x, y] in the L0 gradient image. I_(x) ¹[x, y] denotes ahorizontal gradient value of a pixel position [x, y] in the L1 gradientimage. I_(y) ⁰[x, y] denotes a vertical gradient value of the pixelposition [x, y] in the L0 gradient image. I_(y) ¹[x, y] denotes avertical gradient value of the pixel position [x, y] in the L1 gradientimage.

Moreover, in equation (3), I⁰[x, y] denotes a pixel value of a pixelposition [x, y] in the L0 prediction image. I¹[x, y] denotes a pixelvalue of a pixel position [x, y] in the L1 prediction image. ΔI[x, y]denotes a different between the pixel value of the pixel position [x, y]in the L0 prediction image and the pixel value of the pixel position [x,y] in the L1 prediction image.

Moreover, in equation (3), Ω denotes, for example, a set of pixelpositions included in a region having the pixel position [x, y] at thecenter. w[i, j] denotes a weight coefficient for a pixel position [i,j]. The same value may be used for w[i, j]. G_(x)[x, y], G_(y)[x, y],G_(x)G_(y)[x, y], sG_(x)G_(y)[x, y], sG_(x) ²[x, y], sG_(y) ²[x, y],sG_(x)dI[x, y], sG_(y)dI[x, y], etc. are supplemental calculated values.

Moreover, in equation (3), u[x, y] denotes a horizontal value includedin a local motion estimation value of a pixel position [x, y], v[x, y]denotes a vertical value included in the local motion estimation valueof the pixel position [x, y]. b[x, y] denotes a pixel correction valueof the pixel position [x, y]. p[x, y] denotes a prediction pixel valueof the pixel position [x, y].

Although inter predictor 126 derives the local motion estimation valuefor each pixel in the above description, inter predictor 126 may derivea local motion estimation value for each sub-block that is a unit ofdata more coarse than the pixel and finer than the current block.

For example, in above equation (3), Ω may indicate a set of pixelpositions included in a sub-block. In addition, sG_(x)G_(y)[x, y],sG_(x) ²[x, y], sG_(y) ²[x, y], sG_(x)d[x, y], sG_(y)dI[x, y], u[x, y],and v[x, y] may be calculated not for each pixel but for each sub-block.

Encoder 100 and decoder 200 can apply common BIO. In other words,encoder 100 and decoder 200 can apply BO in the same manner.

[Implementation Example of Encoder]

FIG. 19 is a block diagram illustrating an implementation example ofencoder 100 according to Embodiment 1. Encoder 100 includes circuitry160 and memory 162. For example, the plurality of constituent elementsof encoder 100 illustrated in FIG. 1 and FIG. 11 are implemented bycircuitry 160 and memory 162 illustrated in FIG. 19.

Circuitry 160 performs information processing and is accessible tomemory 162. For example, circuitry 160 is exclusive or generalelectronic circuitry that encodes a video. Circuitry 160 may be aprocessor such as a central processing unit (CPU). Circuitry 160 may bean aggregate of a plurality of electronic circuits. Moreover, forexample, circuitry 160 may perform the functions of two or moreconstituent elements among the plurality of constituent elements ofencoder 100 illustrated in FIG. 1 etc., except the constituent elementsthat store information.

Memory 162 is an exclusive or general memory for storing informationused by circuitry 160 to encode a video. Memory 162 may be an electroniccircuit, may be connected to circuitry 160, or may be included incircuitry 160. Memory 162 may be an aggregate of a plurality ofelectronic circuits. Memory 162 may be a magnetic disc, an optical disc,or the like, or may be expressed as storage, a recording medium, or thelike. Memory 162 may be a non-volatile memory or a volatile memory.

For example, memory 162 may store a video to be encoded, or may store abitstream corresponding to an encoded video. Memory 162 may store aprogram for causing circuitry 1.60 to encode a video.

Moreover, for example, memory 162 may perform the functions of, amongthe plurality of constituent elements of encoder 100 illustrated in FIG.1 etc., the constituent elements that store information. Specifically,memory 162 may perform the functions of block memory 118 and framememory 122 illustrated in FIG. 1. More specifically, memory 162 maystore a reconstructed block, a reconstructed picture, etc.

It should be noted that not all of the plurality of constituent elementsillustrated in FIG. 1 etc. need to be implemented by encoder 100, andnot all of the processes described above need to be performed by encoder100. Some of the constituent elements illustrated in FIG. 1 etc. may beincluded in another device, and some of the processes described abovemay be performed by another device. Encoder 100 performs predictionefficiently by implementing some of the constituent elements illustratedin FIG. 1 etc. and performing some of the processes described above.

Specifically, encoder 100 derives a first motion vector in a unit of aprediction block using a first inter frame prediction mode that uses adegree of matching between two reconstructed images of two regions intwo different pictures, the prediction block being obtained by splittingan image included in a video (S104 or S106 in FIG. 13). Here, the firstinter frame prediction mode is, for example, the above-described FRUCmode. In particular, the first inter frame prediction mode includes atleast one of the template FRUC mode and the bilateral FRUC mode. Inother words, the two regions in the first inter frame prediction modemay be (i) a region in a current picture neighboring a currentprediction block and a region in a reference picture or (ii) two regionsin two different reference pictures.

To put it differently, the first inter frame prediction mode derives amotion vector by the same method between the encoding side and thedecoding side. Moreover, in the first inter frame prediction mode,information indicating a motion vector is not signaled into an encodedstream, and is not transmitted from the encoding side to the decodingside. Furthermore, in the first inter frame prediction mode, encoder 100derives a motion vector using a pixel value of an encoded predictionblock but not a pixel value of a current block.

Next, encoder 100 performs, in the unit of the prediction block, a firstmotion compensation process that generates a prediction image byreferring to a spatial gradient of luminance in an image generated byperforming motion compensation using the derived first motion vector(S103A in FIG. 13). Here, the first motion compensation process is, forexample, above-described BIO, and includes correction using a luminancegradient. Moreover, in the first motion compensation process, aprediction image is corrected in a unit finer than a prediction block(e.g., a unit of a pixel or a unit of a block). Furthermore, in thefirst motion compensation process, a prediction image is generated usinga region in a reference picture indicated by a motion vector, and apixel surrounding the region.

According to this, encoder 100 can reduce an amount of processing byperforming the derivation of the motion vector using the first interframe prediction mode, and the first motion compensation process in theunit of the prediction block, compared to a case in which theseprocesses are performed in a unit of a sub-block. Further, since thefirst compensation process including the correction using the luminancegradient can make correction in a unit finer than the unit of theprediction block, it is possible to suppress a decrease in codingefficiency when the processing is not performed in the unit of thesub-block. Accordingly, encoder 100 can reduce the amount of processingwhile suppressing a decrease in coding efficiency.

Moreover, encoder 100 derives a second motion vector in the unit of theprediction block using a second inter frame prediction mode that uses adegree of matching between a current prediction block and areconstructed image of a region included in a reference picture (S1.02in FIG. 13). Then, encoder 100 generates an encoded bitstream includinginformation for identifying the second motion vector.

Here, the second inter frame prediction mode is, for example, theabove-described normal inter frame prediction mode. To put itdifferently, the second inter frame prediction mode derives a motionvector by a different method between the encoding side and the decodingside. Specifically, encoder 100 derives a motion vector using a pixelvalue of an encoded prediction block, and a pixel value of a currentprediction block. After that, encoder 100 signals information indicatingthe derived motion vector into an encoded stream. With this, theinformation indicating the motion vector derived by encoder 100 istransmitted from encoder 100 to decoder 200. Decoder 200 derives themotion vector using the information included in the encoded stream.

Next, encoder 100 performs, in the unit of the prediction block, asecond motion compensation process that generates a prediction image byreferring to a spatial gradient of luminance in an image generated byperforming motion compensation using the derived second motion vector(S103A in FIG. 13). Here, the second motion compensation process is, forexample, above-described BIO, and includes correction using a luminancegradient. Moreover, in the second motion compensation process, aprediction image is corrected in a unit finer than a prediction block(e.g., a unit of a pixel or a unit of a block). Furthermore, in thesecond motion compensation process, a prediction image is generatedusing a region in a reference picture indicated by a motion vector, anda pixel surrounding the region.

It should be noted that the second motion compensation process may beidentical to the first motion compensation process, or may be partiallydifferent from the first motion compensation process.

With this, it is possible to make a processing unit of motioncompensation identical between when the first inter frame predictionmode is used and when the second inter frame prediction mode is used.Accordingly, it is possible to simplify the implementation of motioncompensation.

[Implementation Example of Decoder]

FIG. 20 is a block diagram illustrating an implementation example ofdecoder 200 according to Embodiment 1. Decoder 200 includes circuitry260 and memory 262. For example, the plurality of constituent elementsof decoder 200 illustrated in FIG. 10 and FIG. 12 are implemented bycircuitry 260 and memory 262 illustrated in FIG. 20.

Circuitry 260 performs information processing and is accessible tomemory 262. For example, circuitry 260 is exclusive or generalelectronic circuitry that decodes a video. Circuitry 260 may be aprocessor such as a central processing unit (CPU). Circuitry 260 may bean aggregate of a plurality of electronic circuits. Moreover, forexample, circuitry 260 may perform the functions of two or moreconstituent elements among the plurality of constituent elements ofdecoder 200 illustrated in FIG. 10 etc., except the constituent elementsthat store information.

Memory 262 is an exclusive or general memory for storing informationused by circuitry 260 to decode a video. Memory 262 may be an electroniccircuit, may be connected to circuitry 260, or may be included incircuitry 260. Memory 262 may be an aggregate of a plurality ofelectronic circuits. Memory 262 may be a magnetic disc, an optical disc,or the like, or may be expressed as storage, a recording medium, or thelike. Memory 262 may be a non-volatile memory or a volatile memory.

For example, memory 262 may store a bitstream corresponding to anencoded video, or may store a video corresponding to a decodedbitstream. Memory 262 may store a program for causing circuitry 260 todecode a video.

Moreover, for example, memory 262 may perform the functions of, amongthe plurality of constituent elements of decoder 200 illustrated in FIG.etc., the constituent elements that store information. Specifically,memory 262 may perform the functions of block memory 210 and framememory 214 illustrated in FIG. 10. More specifically, memory 262 maystore a reconstructed block, a reconstructed picture, etc.

It should be noted that not all of the plurality of constituent elementsillustrated in FIG. 10 etc. need to be implemented by decoder 200, andnot all of the processes described above need to be performed by decoder200. Some of the constituent elements illustrated in FIG. 10 etc. may beincluded in another device, and some of the processes described abovemay be performed by another device. Decoder 200 performs motioncompensation efficiently by implementing some of the constituentelements illustrated in FIG. 10 etc. and performing some of theprocesses described above.

Specifically, decoder 200 derives a first motion vector in a unit of aprediction block using a first inter frame prediction mode that uses adegree of matching between two reconstructed images of two regions intwo different pictures, the prediction block being obtained by splittingan image included in a video (S104 or S106 in FIG. 13). Here, the firstinter frame prediction mode is, for example, the above-described FRUCmode. In particular, the first inter frame prediction mode includes atleast one of the template FRUC mode and the bilateral FRUC mode. Inother words, the two regions in the first inter frame prediction modemay be (i) a region in a current picture neighboring a currentprediction block and a region in a reference picture or (ii) two regionsin two different reference pictures.

To put it differently, the first inter frame prediction mode derives amotion vector by the same method between the encoding side and thedecoding side. Moreover, in the first inter frame prediction mode,information indicating a motion vector is not signaled into an encodedstream, and is not transmitted from the encoding side to the decodingside. Furthermore, in the first inter frame prediction mode, decoder 200derives a motion vector using a pixel value of an encoded predictionblock but not a pixel value of a current block.

Next, decoder 200 performs, in the unit of the prediction block, a firstmotion compensation process that generates a prediction image byreferring to a spatial gradient of luminance in an image generated byperforming motion compensation using the derived first motion vector(S103A in FIG. 13). Here, the first motion compensation process is, forexample, above-described BIO, and includes correction using a luminancegradient. Moreover, in the first motion compensation process, aprediction image is corrected in a unit finer than a prediction block(e.g., a unit of a pixel or a unit of a block). Furthermore, in thefirst motion compensation process, a prediction image is generated usinga region in a reference picture indicated by a motion vector, and apixel surrounding the region.

According to this, decoder 200 can reduce an amount of processing byperforming the derivation of the motion vector using the first interframe prediction mode, and the first motion compensation process in theunit of the prediction block, compared to a case in which theseprocesses are performed in a unit of a sub-block. Further, since thefirst compensation process including the correction using the luminancegradient can make correction in a unit finer than the unit of theprediction block, it is possible to suppress a decrease in codingefficiency when the processing is not performed in the unit of thesub-block. Accordingly, decoder 200 can reduce the amount of processingwhile suppressing a decrease in coding efficiency.

Moreover, decoder 200 obtains, from an encoded bitstream, informationfor identifying a second motion vector in the unit of the predictionblock. Decoder 200 derives the second motion vector in the unit of theprediction block using a second inter frame prediction mode that usesthe information (S102 in FIG. 13).

Here, the second inter frame prediction mode is, for example, theabove-described normal inter frame prediction mode. To put itdifferently, the second inter frame prediction mode derives a motionvector by a different method between the encoding side and the decodingside. Specifically, encoder 100 derives a motion vector using a pixelvalue of an encoded prediction block, and a pixel value of a currentprediction block. After that, encoder 100 signals information indicatingthe derived motion vector into an encoded stream. With this, theinformation indicating the motion vector derived by encoder 100 istransmitted from encoder 100 to decoder 200. Decoder 200 derives themotion vector using the information included in the encoded stream.

Next, decoder 200 performs, in the unit of the prediction block, asecond motion compensation process that generates a prediction image byreferring to a spatial gradient of luminance in an image generated byperforming motion compensation using the derived second motion vector(S103A in FIG. 13). Here, the second motion compensation process is, forexample, above-described BIO, and includes correction using a luminancegradient. Moreover, in the second motion compensation process, aprediction image is corrected in a unit finer than a prediction block(e.g., a unit of a pixel or a unit of a block). Furthermore, in thesecond motion compensation process, a prediction image is generatedusing a region in a reference picture indicated by a motion vector, anda pixel surrounding the region.

It should be noted that the second motion compensation process may beidentical to the first motion compensation process, or may be partiallydifferent from the first motion compensation process.

With this, it is possible to make a processing unit of motioncompensation identical between when the first inter frame predictionmode is used and when the second inter frame prediction mode is used.Accordingly, it is possible to simplify the implementation of motioncompensation.

[Supplemental Information]

Encoder 100 and decoder 200 according to the present embodiment may beused respectively as an image encoder and an image decoder, or may beused respectively as a video encoder and a video decoder. Alternatively,encoder 100 and decoder 200 can be each used as an inter predictiondevice (inter frame prediction device).

In other words, encoder 100 and decoder 200 may correspond only to interpredictor (inter frame predictor) 126 and inter predictor (inter framepredictor) 218, respectively. Other constituent elements such astransformer 106 and inverse transformer 206 may be included in anotherdevice.

In the present embodiment, each of the constituent elements may beconfigured in the form of an exclusive hardware product, or may beimplemented by executing a software program suitable for the constituentelement. Each of the constituent elements may be implemented by means ofa program execution unit, such as a CPU or a processor, reading andexecuting a software program recorded on a recording medium such as ahard disk or a semiconductor memory.

More specifically, encoder 100 and decoder 200 may each includeprocessing circuitry and storage electrically connected to theprocessing circuitry and accessible from the processing circuitry. Forexample, the processing circuitry corresponds to circuitry 160 or 260,and the storage corresponds to memory 162 or 262.

The processing circuitry includes at least one of an exclusive hardwareproduct and a program execution unit, and performs processing using thestorage. When the processing circuitry includes a program executionunit, the storage stores a software program executed by the programexecution unit.

Here, the software for implementing, for example, encoder 100 or decoder200 in the present embodiment includes a program as indicated below.

The constituent elements may be circuits as described above. Thecircuits may constitute circuitry as a whole, or may be individualcircuits. Each constituent element may be implemented by a generalprocessor, or may be implemented by an exclusive processor.

Moreover, processing executed by a particular constituent element may beexecuted by another constituent element. The processing execution ordermay be modified, or a plurality of processes may be executed inparallel. Furthermore, an encoding and decoding device may includeencoder 100 and decoder 200.

Although some aspects of encoder 100 and decoder 200 have been describedabove based on the embodiment, the aspects of encoder 100 and decoder200 are not limited to this embodiment. Various modifications to thepresent embodiment that are conceivable to those skilled in the art, aswell as embodiments resulting from combinations of constituent elementsin different embodiments, may be included within the scope of theaspects of encoder 100 and decoder 200, so long as they do not departfrom the essence of the present disclosure.

The present aspect may be implemented in combination with one or more ofthe other aspects in the present disclosure. In addition, part of theprocesses in the flow charts, part of the constituent elements of thedevices, and part of the syntax described in the present aspect may beimplemented in combination with the other aspects.

Embodiment 2

As described in each of the above embodiments, each functional block cantypically be realized as an MPU and memory, for example. Moreover,processes performed by each of the functional blocks are typicallyrealized by a program execution unit, such as a processor, reading andexecuting software (a program) recorded on a recording medium such asROM. The software may be distributed via, for example, downloading, andmay be recorded on a recording medium such as semiconductor memory anddistributed. Note that each functional block can, of course, also berealized as hardware (dedicated circuit).

Moreover, the processing described in each of the embodiments may berealized via integrated processing using a single apparatus (system),and, alternatively, may be realized via decentralized processing using aplurality of apparatuses. Moreover, the processor that executes theabove-described program may be a single processor or a plurality ofprocessors. In other words, integrated processing may be performed, and,alternatively, decentralized processing may be performed.

Embodiments of the present disclosure are not limited to the aboveexemplary embodiments; various modifications may be made to theexemplary embodiments, the results of which are also included within thescope of the embodiments of the present disclosure.

Next, application examples of the moving picture encoding method (imageencoding method) and the moving picture decoding method (image decodingmethod) described in each of the above embodiments and a system thatemploys the same will be described. The system is characterized asincluding an image encoder that employs the image encoding method, animage decoder that employs the image decoding method, and an imageencoder/decoder that includes both the image encoder and the imagedecoder. Other configurations included in the system may be modified ona case-by-case basis.

[Usage Examples]

FIG. 21 illustrates an overall configuration of content providing systemex100 for implementing a content distribution service. The area in whichthe communication service is provided is divided into cells of desiredsizes, and base stations ex106, ex107, ex108, ex109, and ex110, whichare fixed wireless stations, are located in respective cells.

In content providing system ex100, devices including computer ex111,gaming device ex112, camera ex113, home appliance ex114, and smartphoneex115 are connected to internet ex101 via internet service providerex102 or communications network ex104 and base stations ex106 throughex110. Content providing system ex100 may combine and connect anycombination of the above elements. The devices may be directly orindirectly connected together via a telephone network or near fieldcommunication rather than via base stations ex106 through ex110, whichare fixed wireless stations. Moreover, streaming server ex103 isconnected to devices including computer ex111, gaming device ex112,camera ex113, home appliance ex114, and smartphone ex115 via, forexample, internet ex101. Streaming server ex103 is also connected to,for example, a terminal in a hotspot in airplane ex117 via satelliteex116.

Note that instead of base stations ex106 through ex110, wireless accesspoints or hotspots may be used. Streaming server ex103 may be connectedto communications network ex104 directly instead of via internet ex101or internet service provider ex102, and may be connected to airplaneex117 directly instead of via satellite ex116.

Camera ex113 is a device capable of capturing still images and video,such as a digital camera. Smartphone ex115 is a smartphone device,cellular phone, or personal handyphone system (PHS) phone that canoperate under the mobile communications system standards of the typical2G, 3G, 3.9G, and 4G systems, as well as the next-generation 50 system.

Home appliance ex118 is, for example, a refrigerator or a deviceincluded in a home fuel cell cogeneration system.

In content providing system ex100, a terminal including an image and/orvideo capturing function is capable of, for example, live streaming byconnecting to streaming server ex103 via, for example, base stationex106. When live streaming, a terminal (e.g., computer ex111, gamingdevice ex112, camera ex113, home appliance ex114, smartphone ex1.15, orairplane ex117) performs the encoding processing described in the aboveembodiments on still-image or video content captured by a user via theterminal, multiplexes video data obtained via the encoding and audiodata obtained by encoding audio corresponding to the video, andtransmits the obtained data to streaming server ex103. In other words,the terminal functions as the image encoder according to one aspect ofthe present disclosure.

Streaming server ex103 streams transmitted content data to clients thatrequest the stream. Client examples include computer ex111, gamingdevice ex112, camera ex113, home appliance ex114, smartphone ex1.5, andterminals inside airplane ex117, which are capable of decoding theabove-described encoded data. Devices that receive the streamed datadecode and reproduce the received data. In other words, the devices eachfunction as the image decoder according to one aspect of the presentdisclosure.

[Decentralized Processing]

Streaming server ex103 may be realized as a plurality of servers orcomputers between which tasks such as the processing, recording, andstreaming of data are divided. For example, streaming server ex103 maybe realized as a content delivery network (CDN) that streams content viaa network connecting multiple edge servers located throughout the world.In a CDN, an edge server physically near the client is dynamicallyassigned to the client. Content is cached and streamed to the edgeserver to reduce load times. In the event of, for example, some kind ofan error or a change in connectivity due to, for example, a spike intraffic, it is possible to stream data stably at high speeds since it ispossible to avoid affected parts of the network by, for example,dividing the processing between a plurality of edge servers or switchingthe streaming duties to a different edge server, and continuingstreaming.

Decentralization is not limited to just the division of processing forstreaming; the encoding of the captured data may be divided between andperformed by the terminals, on the server side, or both. In one example,in typical encoding, the processing is performed in two loops. The firstloop is for detecting how complicated the image is on a frame-by-frameor scene-by-scene basis, or detecting the encoding load. The second loopis for processing that maintains image quality and improves encodingefficiency. For example, it is possible to reduce the processing load ofthe terminals and improve the quality and encoding efficiency of thecontent by having the terminals perform the first loop of the encodingand having the server side that received the content perform the secondloop of the encoding. In such a case, upon receipt of a decodingrequest, it is possible for the encoded data resulting from the firstloop performed by one terminal to be received and reproduced on anotherterminal in approximately real time. This makes it possible to realizesmooth, real-time streaming.

In another example, camera ex113 or the like extracts a feature amountfrom an image, compresses data related to the feature amount asmetadata, and transmits the compressed metadata to a server. Forexample, the server determines the significance of an object based onthe feature amount and changes the quantization accuracy accordingly toperform compression suitable for the meaning of the image. Featureamount data is particularly effective in improving the precision andefficiency of motion vector prediction during the second compressionpass performed by the server. Moreover, encoding that has a relativelylow processing load, such as variable length coding (VLC), may behandled by the terminal, and encoding that has a relatively highprocessing load, such as context-adaptive binary arithmetic coding(CABAC), may be handled by the server.

In yet another example, there are instances in which a plurality ofvideos of approximately the same scene are captured by a plurality ofterminals in, for example, a stadium, shopping mall, or factory. In sucha case, for example, the encoding may be decentralized by dividingprocessing tasks between the plurality of terminals that captured thevideos and, if necessary, other terminals that did not capture thevideos and the server, on a per-unit basis. The units may be, forexample, groups of pictures (GOP), pictures, or tiles resulting fromdividing a picture. This makes it possible to reduce load times andachieve streaming that is closer to real-time.

Moreover, since the videos are of approximately the same scene,management and/or instruction may be carried out by the server so thatthe videos captured by the terminals can be cross-referenced. Moreover,the server may receive encoded data from the terminals, change referencerelationship between items of data or correct or replace picturesthemselves, and then perform the encoding. This makes it possible togenerate a stream with increased quality and efficiency for theindividual items of data.

Moreover, the server may stream video data after performing transcodingto convert the encoding format of the video data. For example, theserver may convert the encoding format from MPEG to VP, and may convertH.264 to H.265.

In this way, encoding can be performed by a terminal or one or moreservers. Accordingly, although the device that performs the encoding isreferred to as a “server” or “terminal” in the following description,some or all of the processes performed by the server may be performed bythe terminal, and likewise some or all of the processes performed by theterminal may be performed by the server. This also applies to decodingprocesses.

[3D, Multi-Angle]

In recent years, usage of images or videos combined from images orvideos of different scenes concurrently captured or the same scenecaptured from different angles by a plurality of terminals such ascamera ex113 and/or smartphone ex115 has increased. Videos captured bythe terminals are combined based on, for example, theseparately-obtained relative positional relationship between theterminals, or regions in a video having matching feature points.

In addition to the encoding of two-dimensional moving pictures, theserver may encode a still image based on scene analysis of a movingpicture either automatically or at a point in time specified by theuser, and transmit the encoded still image to a reception terminal.Furthermore, when the server can obtain the relative positionalrelationship between the video capturing terminals, in addition totwo-dimensional moving pictures, the server can generatethree-dimensional geometry of a scene based on video of the same scenecaptured from different angles. Note that the server may separatelyencode three-dimensional data generated from, for example, a pointcloud, and may, based on a result of recognizing or tracking a person orobject using three-dimensional data, select or reconstruct and generatea video to be transmitted to a reception terminal from videos capturedby a plurality of terminals.

This allows the user to enjoy a scene by freely selecting videoscorresponding to the video capturing terminals, and allows the user toenjoy the content obtained by extracting, from three-dimensional datareconstructed from a plurality of images or videos, a video from aselected viewpoint. Furthermore, similar to with video, sound may berecorded from relatively different angles, and the server may multiplex,with the video, audio from a specific angle or space in accordance withthe video, and transmit the result.

In recent years, content that is a composite of the real world and avirtual world, such as virtual reality (VR) and augmented reality (AR)content, has also become popular. In the case of VR images, the servermay create images from the viewpoints of both the left and right eyesand perform encoding that tolerates reference between the two viewpointimages, such as multi-view coding (MVC), and, alternatively, may encodethe images as separate streams without referencing. When the images aredecoded as separate streams, the streams may be synchronized whenreproduced so as to recreate a virtual three-dimensional space inaccordance with the viewpoint of the user.

In the case of AR images, the server superimposes virtual objectinformation existing in a virtual space onto camera informationrepresenting a real-world space, based on a three-dimensional positionor movement from the perspective of the user. The decoder may obtain orstore virtual object information and three-dimensional data, generatetwo-dimensional images based on movement from the perspective of theuser, and then generate superimposed data by seamlessly connecting theimages. Alternatively, the decoder may transmit, to the server, motionfrom the perspective of the user in addition to a request for virtualobject information, and the server may generate superimposed data basedon three-dimensional data stored in the server in accordance with thereceived motion, and encode and stream the generated superimposed datato the decoder. Note that superimposed data includes, in addition to RGBvalues, an a value indicating transparency, and the server sets the avalue for sections other than the object generated fromthree-dimensional data to, for example, 0, and may perform the encodingwhile those sections are transparent. Alternatively, the server may setthe background to a predetermined RGB value, such as a chroma key, andgenerate data in which areas other than the object are set as thebackground.

Decoding of similarly streamed data may be performed by the client(i.e., the terminals), on the server side, or divided therebetween. Inone example, one terminal may transmit a reception request to a server,the requested content may be received and decoded by another terminal,and a decoded signal may be transmitted to a device having a display. Itis possible to reproduce high image quality data by decentralizingprocessing and appropriately selecting content regardless of theprocessing ability of the communications terminal itself. In yet anotherexample, while a TV, for example, is receiving image data that is largein size, a region of a picture, such as a tile obtained by dividing thepicture, may be decoded and displayed on a personal terminal orterminals of a viewer or viewers of the TV. This makes it possible forthe viewers to share a big-picture view as well as for each viewer tocheck his or her assigned area or inspect a region in further detail upclose.

In the future, both indoors and outdoors, in situations in which aplurality of wireless connections are possible over near, mid, and fardistances, it is expected to be able to seamlessly receive content evenwhen switching to data appropriate for the current connection, using astreaming system standard such as MPEG-DASH. With this, the user canswitch between data in real time while freely selecting a decoder ordisplay apparatus including not only his or her own terminal, but also,for example, displays disposed indoors or outdoors. Moreover, based on,for example, information on the position of the user, decoding can beperformed while switching which terminal handles decoding and whichterminal handles the displaying of content. This makes it possible to,while in route to a destination, display, on the wall of a nearbybuilding in which a device capable of displaying content is embedded oron part of the ground, map information while on the move. Moreover, itis also possible to switch the bit rate of the received data based onthe accessibility to the encoded data on a network, such as when encodeddata is cached on a server quickly accessible from the receptionterminal or when encoded data is copied to an edge server in a contentdelivery service.

[Scalable Encoding]

The switching of content will be described with reference to a scalablestream, illustrated in FIG. 22, that is compression coded viaimplementation of the moving picture encoding method described in theabove embodiments. The server may have a configuration in which contentis switched while making use of the temporal and/or spatial scalabilityof a stream, which is achieved by division into and encoding of layers,as illustrated in FIG. 22. Note that there may be a plurality ofindividual streams that are of the same content but different quality.In other words, by determining which layer to decode up to based oninternal factors, such as the processing ability on the decoder side,and external factors, such as communication bandwidth, the decoder sidecan freely switch between low resolution content and high resolutioncontent while decoding. For example, in a case in which the user wantsto continue watching, at home on a device such as a TV connected to theinternet, a video that he or she had been previously watching onsmartphone ex115 while on the move, the device can simply decode thesame stream up to a different layer, which reduces server side load.

Furthermore, in addition to the configuration described above in whichscalability is achieved as a result of the pictures being encoded perlayer and the enhancement layer is above the base layer, the enhancementlayer may include metadata based on, for example, statisticalinformation on the image, and the decoder side may generate high imagequality content by performing super-resolution imaging on a picture inthe base layer based on the metadata. Super-resolution imaging may beimproving the SN ratio while maintaining resolution and/or increasingresolution. Metadata includes information for identifying a linear or anon-linear filter coefficient used in super-resolution processing, orinformation identifying a parameter value in filter processing, machinelearning, or least squares method used in super-resolution processing.

Alternatively, a configuration in which a picture is divided into, forexample, tiles in accordance with the meaning of, for example, an objectin the image, and on the decoder side, only a partial region is decodedby selecting a tile to decode, is also acceptable. Moreover, by storingan attribute about the object (person, car, ball, etc.) and a positionof the object in the video (coordinates in identical images) asmetadata, the decoder side can identify the position of a desired objectbased on the metadata and determine which tile or tiles include thatobject. For example, as illustrated in FIG. 23, metadata is stored usinga data storage structure different from pixel data such as an SEImessage in HEVC. This metadata indicates, for example, the position,size, or color of the main object.

Moreover, metadata may be stored in units of a plurality of pictures,such as stream, sequence, or random access units. With this, the decoderside can obtain, for example, the time at which a specific personappears in the video, and by fitting that with picture unit information,can identify a picture in which the object is present and the positionof the object in the picture.

[Web Page Optimization]

FIG. 24 illustrates an example of a display screen of a web page on, forexample, computer ex111. FIG. 25 illustrates an example of a displayscreen of a web page on, for example, smartphone ex115. As illustratedin FIG. 24 and FIG. 25, a web page may include a plurality of imagelinks which are links to image content, and the appearance of the webpage differs depending on the device used to view the web page. When aplurality of image links are viewable on the screen, until the userexplicitly selects an image link, or until the image link is in theapproximate center of the screen or the entire image link fits in thescreen, the display apparatus (decoder) displays, as the image links,still images included in the content or I pictures, displays video suchas an animated gif using a plurality of still images or I pictures, forexample, or receives only the base layer and decodes and displays thevideo.

When an image link is selected by the user, the display apparatusdecodes giving the highest priority to the base layer. Note that ifthere is information in the HTML code of the web page indicating thatthe content is scalable, the display apparatus may decode up to theenhancement layer. Moreover, in order to guarantee real timereproduction, before a selection is made or when the bandwidth isseverely limited, the display apparatus can reduce delay between thepoint in time at which the leading picture is decoded and the point intime at which the decoded picture is displayed (that is, the delaybetween the start of the decoding of the content to the displaying ofthe content) by decoding and displaying only forward reference pictures(I picture, P picture, forward reference B picture). Moreover, thedisplay apparatus may purposely ignore the reference relationshipbetween pictures and coarsely decode all B and P pictures as forwardreference pictures, and then perform normal decoding as the number ofpictures received over time increases.

[Autonomous Driving]

When transmitting and receiving still image or video data such two- orthree-dimensional map information for autonomous driving or assisteddriving of an automobile, the reception terminal may receive, inaddition to image data belonging to one or more layers, information on,for example, the weather or road construction as metadata, and associatethe metadata with the image data upon decoding. Note that metadata maybe assigned per layer and, alternatively, may simply be multiplexed withthe image data.

In such a case, since the automobile, drone, airplane, etc., includingthe reception terminal is mobile, the reception terminal can seamlesslyreceive and decode while switching between base stations among basestations ex106 through ex110 by transmitting information indicating theposition of the reception terminal upon reception request. Moreover, inaccordance with the selection made by the user, the situation of theuser, or the bandwidth of the connection, the reception terminal candynamically select to what extent the metadata is received or to whatextent the map information, for example, is updated.

With this, in content providing system ex100, the client can receive,decode, and reproduce, in real time, encoded information transmitted bythe user.

[Streaming of Individual Content]

In content providing system ex100, in addition to high image quality,long content distributed by a video distribution entity, unicast ormulticast streaming of low image quality, short content from anindividual is also possible. Moreover, such content from individuals islikely to further increase in popularity. The server may first performediting processing on the content before the encoding processing inorder to refine the individual content. This may be achieved with, forexample, the following configuration.

In real-time while capturing video or image content or after the contenthas been captured and accumulated, the server performs recognitionprocessing based on the raw or encoded data, such as capture errorprocessing, scene search processing, meaning analysis, and/or objectdetection processing. Then, based on the result of the recognitionprocessing, the server-either when prompted or automatically-edits thecontent, examples of which include: correction such as focus and/ormotion blur correction; removing low-priority scenes such as scenes thatare low in brightness compared to other pictures or out of focus; objectedge adjustment; and color tone adjustment. The server encodes theedited data based on the result of the editing. It is known thatexcessively long videos tend to receive fewer views. Accordingly, inorder to keep the content within a specific length that scales with thelength of the original video, the server may, in addition to thelow-priority scenes described above, automatically clip out scenes withlow movement based on an image processing result. Alternatively, theserver may generate and encode a video digest based on a result of ananalysis of the meaning of a scene.

Note that there are instances in which individual content may includecontent that infringes a copyright, moral right, portrait rights, etc.Such an instance may lead to an unfavorable situation for the creator,such as when content is shared beyond the scope intended by the creator.Accordingly, before encoding, the server may, for example, edit imagesso as to blur faces of people in the periphery of the screen or blur theinside of a house, for example. Moreover, the server may be configuredto recognize the faces of people other than a registered person inimages to be encoded, and when such faces appear in an image, forexample, apply a mosaic filter to the face of the person. Alternatively,as pre- or post-processing for encoding, the user may specify, forcopyright reasons, a region of an image including a person or a regionof the background be processed, and the server may process the specifiedregion by, for example, replacing the region with a different image orblurring the region. If the region includes a person, the person may betracked in the moving picture, and the head region may be replaced withanother image as the person moves.

Moreover, since there is a demand for real-time viewing of contentproduced by individuals, which tends to be small in data size, thedecoder first receives the base layer as the highest priority andperforms decoding and reproduction, although this may differ dependingon bandwidth. When the content is reproduced two or more times, such aswhen the decoder receives the enhancement layer during decoding andreproduction of the base layer and loops the reproduction, the decodermay reproduce a high image quality video including the enhancementlayer. If the stream is encoded using such scalable encoding, the videomay be low quality when in an unselected state or at the start of thevideo, but it can offer an experience in which the image quality of thestream progressively increases in an intelligent manner. This is notlimited to just scalable encoding; the same experience can be offered byconfiguring a single stream from a low quality stream reproduced for thefirst time and a second stream encoded using the first stream as areference.

[Other Usage Examples]

The encoding and decoding may be performed by LSI ex500, which istypically included in each terminal. LSI ex500 may be configured of asingle chip or a plurality of chips. Software for encoding and decodingmoving pictures may be integrated into some type of a recording medium(such as a CD-ROM, a flexible disk, or a hard disk) that is readable by,for example, computer ex111, and the encoding and decoding may beperformed using the software. Furthermore, when smartphone ex115 isequipped with a camera, the video data obtained by the camera may betransmitted. In this case, the video data is coded by LSI ex500 includedin smartphone ex115.

Note that LSI ex500 may be configured to download and activate anapplication. In such a case, the terminal first determines whether it iscompatible with the scheme used to encode the content or whether it iscapable of executing a specific service. When the terminal is notcompatible with the encoding scheme of the content or when the terminalis not capable of executing a specific service, the terminal firstdownloads a codec or application software then obtains and reproducesthe content.

Aside from the example of content providing system ex100 that usesinternet ex101, at least the moving picture encoder (image encoder) orthe moving picture decoder (image decoder) described in the aboveembodiments may be implemented in a digital broadcasting system. Thesame encoding processing and decoding processing may be applied totransmit and receive broadcast radio waves superimposed with multiplexedaudio and video data using, for example, a satellite, even though thisis geared toward multicast whereas unicast is easier with contentproviding system ex100.

[Hardware Configuration]

FIG. 26 illustrates smartphone ex115. FIG. 27 illustrates aconfiguration example of smartphone ex115. Smartphone ex115 includesantenna ex450 for transmitting and receiving radio waves to and frombase station ex110, camera ex465 capable of capturing video and stillimages, and display ex458 that displays decoded data, such as videocaptured by camera ex465 and video received by antenna ex450. Smartphoneex115 further includes user interface ex466 such as a touch panel, audiooutput unit ex457 such as a speaker for outputting speech or otheraudio, audio input unit ex456 such as a microphone for audio input,memory ex467 capable of storing decoded data such as captured video orstill images, recorded audio, received video or still images, and mail,as well as decoded data, and slot ex464 which is an interface for SIMex468 for authorizing access to a network and various data. Note thatexternal memory may be used instead of memory ex467.

Moreover, main controller ex460 which comprehensively controls displayex458 and user interface ex466, power supply circuit ex461, userinterface input controller ex462, video signal processor ex455, camerainterface ex463, display controller ex459, modulator/demodulator ex452,multiplexer/demultiplexer ex453, audio signal processor ex454, slotex464, and memory ex467 are connected via bus ex470.

When the user turns the power button of power supply circuit ex461 on,smartphone ex115 is powered on into an operable state by each componentbeing supplied with power from a battery pack.

Smartphone ex115 performs processing for, for example, calling and datatransmission, based on control performed by main controller ex460, whichincludes a CPU, ROM, and RAM. When making calls, an audio signalrecorded by audio input unit ex456 is converted into a digital audiosignal by audio signal processor ex454, and this is applied with spreadspectrum processing by modulator/demodulator ex452 and digital-analogconversion and frequency conversion processing by transmitter/receiverex451, and then transmitted via antenna ex450. The received data isamplified, frequency converted, and analog-digital converted, inversespread spectrum processed by modulator/demodulator ex452, converted intoan analog audio signal by audio signal processor ex454, and then outputfrom audio output unit ex457. In data transmission mode, text,still-image, or video data is transmitted by main controller ex460 viauser interface input controller ex462 as a result of operation of, forexample, user interface ex466 of the main body, and similar transmissionand reception processing is performed. In data transmission mode, whensending a video, still image, or video and audio, video signal processorex455 compression encodes, via the moving picture encoding methoddescribed in the above embodiments, a video signal stored in memoryex467 or a video signal input from camera ex465, and transmits theencoded video data to multiplexer/demultiplexer ex453. Moreover, audiosignal processor ex454 encodes an audio signal recorded by audio inputunit ex456 while camera ex465 is capturing, for example, a video orstill image, and transmits the encoded audio data tomultiplexer/demultiplexer ex453. Multiplexer/demultiplexer ex453multiplexes the encoded video data and encoded audio data using apredetermined scheme, modulates and converts the data usingmodulator/demodulator (modulator/demodulator circuit) ex452 andtransmitter/receiver ex451, and transmits the result via antenna ex450.

When video appended in an email or a chat, or a video linked from a webpage, for example, is received, in order to decode the multiplexed datareceived via antenna ex450, multiplexer/demultiplexer ex453demultiplexes the multiplexed data to divide the multiplexed data into abitstream of video data and a bitstream of audio data, supplies theencoded video data to video signal processor ex455 via synchronous busex470, and supplies the encoded audio data to audio signal processorex454 via synchronous bus ex470. Video signal processor ex455 decodesthe video signal using a moving picture decoding method corresponding tothe moving picture encoding method described in the above embodiments,and video or a still image included in the linked moving picture file isdisplayed on display ex458 via display controller ex459. Moreover, audiosignal processor ex454 decodes the audio signal and outputs audio fromaudio output unit ex457. Note that since real-time streaming is becomingmore and more popular, there are instances in which reproduction of theaudio may be socially inappropriate depending on the user's environment.Accordingly, as an initial value, a configuration in which only videodata is reproduced, i.e., the audio signal is not reproduced, ispreferable. Audio may be synchronized and reproduced only when an input,such as when the user clicks video data, is received.

Although smartphone ex115 was used in the above example, threeimplementations are conceivable: a transceiver terminal including bothan encoder and a decoder; a transmitter terminal including only anencoder; and a receiver terminal including only a decoder. Further, inthe description of the digital broadcasting system, an example is givenin which multiplexed data obtained as a result of video data beingmultiplexed with, for example, audio data, is received or transmitted,but the multiplexed data may be video data multiplexed with data otherthan audio data, such as text data related to the video. Moreover, thevideo data itself rather than multiplexed data maybe received ortransmitted.

Although main controller ex460 including a CPU is described ascontrolling the encoding or decoding processes, terminals often includeGPUs. Accordingly, a configuration is acceptable in which a large areais processed at once by making use of the performance ability of the GPUvia memory shared by the CPU and GPU or memory including an address thatis managed so as to allow common usage by the CPU and GPU. This makes itpossible to shorten encoding time, maintain the real-time nature of thestream, and reduce delay. In particular, processing relating to motionestimation, deblocking filtering, sample adaptive offset (SAO), andtransformation/quantization can be effectively carried out by the GPUinstead of the CPU in units of, for example pictures, all at once.

The present aspect may be implemented in combination with one or more ofthe other aspects in the present disclosure. In addition, part of theprocesses in the flow charts, part of the constituent elements of thedevices, and part of the syntax described in the present aspect may beimplemented in combination with the other aspects.

Although only some exemplary embodiments of the present disclosure havebeen described in detail above, those skilled in the art will readilyappreciate that many modifications are possible in the exemplaryembodiments without materially departing from the novel teachings andadvantages of the present disclosure. Accordingly, all suchmodifications are intended to be included within the scope of thepresent disclosure.

INDUSTRIAL APPLICABILITY

The present disclosure is applicable to, for example, televisionreceivers, digital video recorders, car navigation systems, mobilephones, digital cameras, digital video cameras, teleconferencingsystems, electronic mirrors, etc.

1-8. (canceled)
 9. An encoder comprising: memory; and a processorcoupled to the memory and configured to: perform a first process toderive a first motion vector of a current block; and generate aprediction image of the current block by referring to a spatial gradientof luminance generated by performing motion compensation using the firstmotion vector derived, wherein the first motion vector is not encoded ina bitstream and the first motion vector is not modified per sub-block,the sub-block being obtained by splitting the current block, a secondprocess, which is the same as the first process, is performed in adecoder to derive the first motion vector, and in the first process,evaluation values are calculated for each candidate for the first motionvector, and a motion vector having a better evaluation value than otherevaluation values is determined as the first motion vector, each of theevaluation values corresponding to a difference between two regions intwo reconstructed images.
 10. A decoder comprising: memory; and aprocessor coupled to the memory and configured to: perform a secondprocess to derive a first motion vector of a current block; and generatea prediction image of the current block by referring to a spatialgradient of luminance generated by performing motion compensation usingthe first motion vector derived, wherein the first motion vector is notencoded in a bitstream and the first motion vector is not modified persub-block, the sub-block being obtained by splitting the current block,a first process, which is the same as the second process, is performedin an encoder to derive the first motion vector, and in the secondprocess, evaluation values are calculated for each candidate for thefirst motion vector, and a motion vector having a better evaluationvalue than other evaluation values is determined as the first motionvector, each of the evaluation values corresponding to a differencebetween two regions in two reconstructed images.